American Mediums
The popular history of the American bomber forces during WWII is dominated by that of the heavy bomber forces, which were equipped with the B-17 and B-24, and late in the war the even larger B-29 and smaller numbers of the B-32. The historical importance of the heavy bomber force cannot be denied, and it is very significant that the big B-24 was the US combat aircraft with the largest production run. From 1939 onwards, strategy was constructed around bombardment of the enemy’s industrial and economic basis. It helped that Germany was, in the end, a compact country, where all targets were within reach of heavy bombers based in England. Japan too was limited in geographical extent, but because it was distant, the conquest of bomber bases within attack distance became a strategic goal in itself. Besides maintaining large-scale B-17 and B-24 production, the USA invested enormous industrial resources in the development of the B-29. Even the development of an intercontinental heavy bomber, later entering service as the B-36, was already on the cards.
But American medium bombers were built in substantial numbers too. The USAAF arguably benefited from having twin-engined medium bombers in three major ways. First, they were a cost-effective way to attack targets that were smaller or less distant: In the ETO the medium bombers often used fundamentally the same bombing tactics as the heavy bombers, but they were sent after a different category of targets. Second, when accuracy called for attacks to be made from low or medium altitude, for example against bridges or ships, the medium bombers could do so more effectively and with less risk. American heavy bombers were nearly all fitted with turbocharged engines to operate efficiently at high altitude, but the engines of medium bombers were optimised to operate at low and medium altitude. B-17 attacks against enemy ships turned out to be hopelessly ineffective, but the B-25 evolved into a deadly anti-shipping weapon. And thirdly, at the end of long supply chains such as that to the South-West Pacific, China or Burma, it was easier to build bases for medium bombers than for heavy bombers, and more affordable to keep them supplied. They needed smaller airfields and smaller crews.
Some of the aircraft discussed here did not carry the B for bomber designation, but the A for attack letter. Originally most of the USAAC aircraft in the Attack category had been single-engined, light bombers or dive bombers. But as tactical thinking evolved towards interdicting enemy forces and supplies on their way to the battlefield, instead of attacking them on the battlefield itself, it naturally came to favour heavier and longer-ranged twin-engined attack aircraft. Hence the distinction with bombers became blurred, and a few types even changed designation across the boundary. In practice the B-25 Mitchell was often used in a low-level attack role, while the A-20 Boston might fly medium-altitude level-bombing missions. In the case of the A-26 Invader, designation changes from A-26 to B-26 and back to A-26 were in part motivated by a desire to obfuscate and cause confusion, which they did.
Another source of designation confusion was the use of these bombers by the US Navy. The USA largely managed to avoid the wasteful complexity that grew in Japan, where the Army and Navy developed entirely separate bomber types. The US Army was initially unwilling to allow the Navy to operate multi-engined bomber types from land bases, but it later traded its agreement that the Navy could operate medium bombers in exchange for the industrial resources that the Army needed to build heavy bombers, in particular the B-29. As a result, the Navy and the Marine Corps acquired derivatives of Army Air Force aircraft, to which they gave their own designations. Hence variants of the B-25 became known as the PBJ, for example.
As we have seen, before the USA entered the war, American aircraft manufacturers benefited from substantial British and French orders, which paid for a large expansion of industrial plant and fed in relevant combat experience. Both bombers that were part of their initial orders, the Martin 167 and the Douglas DB-7, underwent further development, in part at the request of these customers. A common factor in these developments was the availability of the Wright R-2600 engine, a 14-cylinder, twin-row radial. This was compact enough to allow an upgrade of existing bombers. The USAAC also ordered a development of the Douglas B-18 with R-2600 engines as the B-23, but due to the obsolescence of this airframe the B-23 had only a small production run.
Baltimore
More important was the Martin model 187 Baltimore, an improvement of the model 167 Maryland with a deeper fuselage, R-2600 engines, and modified tail surfaces. Started in May 1940 as an Anglo-French project, it was taken over fully by Britain after the fall of France. Development of the Baltimore overlapped with that of the Martin model 179, i.e. the B-26 Marauder, which was recognised to be a superior design. The project only continued because it had progressed too far to be cancelled, and the Baltimore became a less important type in the Allied inventory. The RAF nevertheless took delivery of 1575 of them. The five production versions were distinguished mostly by variations in their defensive armament, and by the fact that later models were built to US orders (as the A-30) and delivered to the RAF under Lend-Lease. The Baltimore served successfully, mainly in the Mediterranean theatre. But as it was nevertheless somewhat surplus to requirements, 71 aircraft were delivered to Turkey, and other aircraft equipped squadrons manned by Free French, Greek, or Italian crews.
Boston
The trim, businesslike Douglas DB-7 was also a good candidate for fitting the R-2600 engine. Douglas accordingly built the DB-7A to French specifications, the DB-7B to British specifications, and the A-20 to US Air Corps specifications. The British wanted changes in internal layout, better armament, and self-sealing fuel tanks. The USAAC project initially envisaged the use of turbo-supercharged R-2600 engines to create a high-altitude light bomber. However, as these engines were underdeveloped and requirements changed, the turbocharger was omitted from the production A-20A. Confusingly, the A-20A was similar to the DB-7B and the A-20B to the DB-7A. In early 1941, a common British and American production standard was established as the A-20C. The British named their bomber version “Boston” and used the name “Havoc” for their intruder and nightfighter versions. The Americans adopted the name “Havoc” for the A-20 bomber and developed their own nightfighter version, which was known as the P-70. The USSR had expressed interest in the type as early as September 1939, but started to receive the type as Lend-Lease equipment in December 1941. They assigned the official designation B-3 and received about 3000 of these aircraft (the number varies significantly between sources).
Compared to the original DB-7, the A-20A not only had more powerful engines. It had a stronger structure, a longer nose, a bigger tail, and substantially more fuel capacity. It was also heavier, with take-off weight increased from 6,872 to 8,641 kg. But it was still much smaller and lighter than the B-25 with similar R-2600 engines, and had a crew of three. Thanks to a relatively small size and aerodynamic efficiency, the A-20A was fast, with a top speed of 563 km/h at 4000 m. On the other hand, bomb load was only 1090 kg. The Soviets would make modifications to increase both the maximum bomb load (to 1600 kg) and the number of smaller bombs that could be carried.
There was much to like about the A-20: It was a fast, rugged, reliable aircraft with light controls and good flying characteristics. The crew benefited from good ergonomics. But its defensive armament was commonly found too light, with only a .30 machine gun each in flexible dorsal and ventral mounts. The A-20 never had a tail gun position, nor did the bombardier in the nose wield a gun. Instead, there were fixed guns in the lower nose, four .30 weapons on the A-20A, or two .50 guns from the A-20B onwards. Two more .30 guns were installed fixed firing rearwards in the aft end of the engine nacelles, but this was an unsuccessful experiment and soon abandoned. In late 1942, the Soviets decided to refit their B-3s with an UTK-1 dorsal turret with a single 12.7-mm Berezin UBT and another UBT in the ventral mount. On the US production lines, the dorsal armament would be improved first by replacing the .30 with a .50 gun, and later (from the A-20G-20-DO onwards) by installing the excellent Martin 250GE turret with two .50 guns. The ventral gun was also replaced by a .50. The Martin turret had a much lower profile than the UTK-1.
From the start, Douglas had planned alternative noses, reflecting different tactical applications. The A-20A, B, C, J and K were built with noses that had a bombardier’s position and transparent paneling, though the lower nose contained fixed guns as mentioned. Such aircraft were more suited for conventional medium-altitude bombing and these noses were fitted to the initial production runs. Combat experience in the South-West Pacific Area indicated that increased forward-firing armament was useful for low-level attacks, and captain Paul “Pappy” Gunn elected to fit four or six .50 guns in an improvised mount. This lead to the production of the A-20G with a “solid” nose and initially four 20-mm cannon, later six .50 machine guns. The A-20G also had additional armour. The A-20G and the similar A-20H appear to have been successful in the Pacific, but the Soviets found the type too vulnerable for low-level attacks on the Eastern front, and converted many back to have a bombardier-navigator position. The USSR converted other A-20Gs to nightfighters, reconnaissance aircraft and torpedo-bombers. In Europe too, the A-20G often operated as a conventional medium-altitude bomber, dropping on the signal of a lead bombardier in an A-20J or K.
Because its performance was good by the standards for a bomber, the type was modified by several of its operators to be a nightfighter, but with generally indifferent results. The RAF found that the Havoc was unsuited as a defensive night interceptor, but was very useful as an “intruder”, attacking the German bombers over their own bases. (The RAF indulged in a number of futile experiments, including the fitting of searchlights to illuminate targets for Hurricanes, and the dropping of anti-aircraft parachute mines.) The USAAF also found their P-70 lacking in the desired performance to intercept Japanese bombers, but the type proved useful to train nightfighter crews. And the USSR had only limited success with a nightfighter conversion equipped with Gneis-2 radar. Like the German Do 17 and Do 217, the A-20 was temptingly close to the performance expected of a nightfighter, but didn’t quite achieve it.
With a production run of 7,385, the Douglas bomber was a significant and successful wartime bomber. Production ended in September 1944 in favour of its successor, the Douglas A-26.
Mitchell
The USA built no less than 9,817 of the North American B-25 Mitchell and its derivates, which included the PBJ for the US Navy, the AT-24 (later TB-25) trainer, and the F-10 reconnaissance aircraft. This made the B-25 the American medium bomber built in the largest numbers. (Almost twice as many heavy B-24 Liberators were delivered.)
It might have remained a prototype. The B-25 was a derivative of the North American NA-40, a twin-engined attack bomber that made its first flight on 29 January 1940 powered by a pair of Pratt & Whitney R-1830 engines. The second prototype, known as the NA-40B, was fitted with Wright R-2600 radials. But no order followed because after the loss of both the NA-40B and the competing Douglas DB-7, the US Air Corps cancelled the competition for a new attack bomber. However, in March it issued a new specification for a medium bomber, which was to have the bomb carrying capacity doubled to 4000 lb. North American responded by offering the NA-62. This was similar in overall configuration to the NA-40, but otherwise a new, larger and heavier aircraft. The new fuselage was not as deep, but longer and wider, which allowed the pilot and co-pilot to sit side-by-side instead of in tandem. The NA-62 also had a mid-wing instead of shoulder-wing configuration. The Air Corps preferred the more advanced Martin model 179, which would become the B-26 Marauder, but because this aircraft could not be delivered in the desired numbers and time frame, it also ordered the North American NA-62. Reflecting the urgency of the times, the contracts for 201 and 184 aircraft respectively were signed before either prototype had flown. The first B-25 would fly in August 1940. North American designed the B-25 for mass production from the start, and it paid off. Even before the USA entered the war, the ambitious expansion plans for the air force needed more bombers, and large orders were placed for aircraft to be built in Inglewood and in a new factory in Kansas City. The latter would deliver the bulk of B-25 production, with 2290 of the B-25D, which was similar to the Inglewood-built B-25C, and 4318 of the B-25J.
As was common on 1930s medium bombers, the initial defensive armament was light, with a .50 gun in a tail position and .30 guns in nose, waist, and dorsal positions. But the B-25 gradually evolved into one of the most heavily armed medium bombers of the war. The B-25B was fitted with armour, self-sealing fuel tanks, a Bendix L dorsal turret, and a Bendix K retractable ventral turret, all other armament being removed except a .30 in a nose mount. Unfortunately the retractable ventral turret was rather unsuccessful. Apart from its technical flaws, the periscopic sighting system was difficult to use, with a too narrow field of view and a disorienting effect on the gunner. Many units removed the turret, and used the weight saving to re-introduce waist gunners or install an extra fuel tank. Later the ventral turret was omitted from production. The dorsal turret was to stay and was replaced by the Bendix N on the B-25C, which also had heavier nose armament, more fuel, and various improvements to its combat equipment.
The modern mind expects any change to aircraft to be subjected to a thorough engineering and safety study and a cartload of paperwork. WWII conditions were somewhat different and it was surprisingly common for service depots and combat units to modify their aircraft, relying on local innovation as well as Army-distributed retrofitting kits. In the case of the B-25C and D, that frequently included the improvisation of a tail gun position, by removing the tail cap and providing a cramped position for a gunner with a single .50. The need for a tail defence filtered back to North American. Another operational armament addition, common in the South-West Pacific, were fixed guns installed in the nose, or blisters attached to the sides of the nose with two .50 guns. This too was introduced into the production line, in the form of nose packs with two or four .50s. The widespread use of the B-25 for anti-shipping operations and low-level strafing attacks, again mostly in the Pacific, resulted in the production of models with a “solid” nose and powerful forward-firing armament. A short-barrelled 75-mm M4 cannon was installed in the nose of the B-25G, with two .50s to help in sighting. The big cannon was a specialised weapon for targets such as small ships, and in absence of such targets it was often removed in the field.
The cumulation of combat experience and feedback from the frontline resulted in the production of the B-25H, a specialised strafer that bristled with guns. Externally, it could be easily recognised by the relocation of the dorsal turret to a position aft of the cockpit in line with the leading edge of the wing, instead of the trailing edge, and the addition of a new tail turret, the Bell M-7 with twin .50 guns. The nose armament consisted of the T13E1 75-mm cannon (lighter than the M4), four .50 guns in the nose, and often four more in external packs attached to the fuselage sides. Redesigned, staggered waist gun positions had a .50 each with blister window to increase crew comfort. The B-25J, a version of the B-25H with a glass “bombardier” nose, entered large scale production at the end of 1943, and would become the most-built model of the B-25, with 4318 delivered. For the B-25J a streamlined solid nose with eight .50 machine guns, different from the B-25H nose, was again provided as alternative to the bombardier nose.
Superficially, all these B-25 models looked very similar. But the increase in fixed armament, and other changes dictated by combat experience, such as an external armour plate to the side of the cockpit, was not compensated by any significant increase in engine power. (An attempt to fit R-2800 engines was abandoned.) The original B-25 had a top speed of 518 km/h at 4600 m but the B-25J was down to 438 km/h. Combat units often operated different models at the same time, which meant that the speed of the formation was dictated by the slowest aircraft. Some units dispensed with the external gun packs to reduce weight and drag, or removed the heavy cannon from the B-25G and B-25H.
As mentioned, a number of aircraft ended up with the US Marine Corps, and the designation PBJ. (PB for Patrol-Bomber, J the manufacturer letter assigned to North American.) Their number was relatively modest at 706 aircraft of different versions, and they were land-based, although one PBJ-1H did make a successful landing on the carrier USS Shangri-La in late 1944.
By any standard the B-25 was a success. It served in all theatres of war, flown by American, British, Russian, Chinese, French, Dutch, Australian and Canadian crews, among others. While it was effectively used as a conventional medium-altitude bomber, arguably its greatest merit was its suitability for low-level attack. It was rugged, very agile for its size, and heavily armed. This made the B-25 a very effective weapon against ships, airfields, bridges, railway junctions, and other precision targets. It’s most famous operation was probably the “Doolittle Raid” of 18 April 1942 which saw 16 B-25Bs launched from the deck of the aircraft carrier USS Hornet to strike targets in Japan. While this attack had more of a morale-boosting impact than anything else, it ensured that the Mitchell would not be forgotten.
Marauder
The Martin B-26 Marauder was designed for the same set of requirements as the North American B-25, but while the B-25 was based on the earlier NA-40 prototype, the B-26 had little in common with the earlier Martin bombers. It was a clean-sheet design characterised by two Pratt & Whitney R-2800 radial engines, a well-streamlined but voluminous fuselage, and relatively small wings with a high wing loading for the time. When assessed by the USAAC, the B-26 design scored far better than the B-25, and as mentioned it was only because Martin could not deliver the total number of aircraft that the Air Corps wanted, that the B-25 did enter production too. This was somewhat ironic as Martin’s initial order for 201 aircraft was quickly overtaken by the construction of two additional factories, financed by Uncle Sam, and additional orders that made the Army’s initial wish for 385 medium bombers look paltry by comparison. A total of 5,266 Marauders would be delivered.
The first B-26 flew on 25 November 1940. It wasn’t a hand-built prototype but a first aircraft to come of the production line, a considerable risk as any major modifications would result in the scrapping of production jigs. The first B-26 unit, the 22nd Bomb Group, received its first aircraft in February 1941. A year later it found itself in Australia, flying low-level attack missions against Japanese ships and ground targets.
The B-26 was a fairly substantial aircraft, bigger than most contemporary medium bombers, weighing about 10 tons empty and approximately 17 tons at maximum take-off weight. Thanks to powerful R-2800 engines and attention to aerodynamics, it was still relatively fast. The original B-26A is credited with a cruise speed of 415 km/h and a maximum speed of 506 km/h. Later models, with more powerful engines but a larger wing and more weight, are attributed an even higher cruise speed of 440 km/h (for the B-26F) although maximum speed was reduced (452 km/h). This was good but not exceptional performance. The Do 217, which is probably the Axis bomber that was closest in overall design characteristics, was at least equivalent in performance and probably, with some variations between models, a bit faster. On paper at least, a Do 217 could also lift a somewhat heavier bomb load over a longer range, but the B-26 did have the option to install extra fuel tanks in the bomb bay. The B-26 did carry heavier defensive armament.
The design priorities came at the cost of a high landing speed (240 km/h), and this became controversial when the rapid expansion of the force confronted novice pilots with a demanding aircraft. Generally speaking, to cruise efficiently at high speed an aircraft should not have a wing larger than necessary. But to fly at lower speeds for take-off and landing, a larger wing is desirable, as at lower speeds a wing will generate less lift per unit of area. On a modern airliner, this conflict is resolved by equipping the wing with a complex set of leading and trailing edge flaps, which are deployed for take-off and landing to increase effective area and help stabilise the airflow. As initially built, the B-26 had a small wing with high loading and only simple flaps. And the weight of the new bomber was increased well above the design weight by the addition of armour plate, self-sealing fuel tanks, increased defensive armament, and other desirable equipment. In combination with freshly trained pilots, this did prove dangerous. A number of fatal accidents attracted political attention and put the B-26 in the spotlights in an unfavourable way. It is often remembered primarily for this, which is rather unfair. Martin introduced more effective, larger area slotted flaps, a longer nosewheel leg (to increase the angle of incidence of the wing), and in early 1943 an enlarged set of wings and tailplanes. Late in 1943 the B-26F and G model also introduced a change in wing incidence relative to the fuselage. All this helped to mitigate the problem.
On the initial B-26A only the Martin 250CE dorsal turret with two .50 guns was a powered mount. A manually aimed .50 was installed in a tail gun position, and .30 guns were fitted as nose and ventral guns. This was insufficient, so later models saw the addition of a fixed nose gun, the addition of waist gunners with a .50 each (initially a .30), the installation of a Martin-Bell M6 powered tail turret with two .50 guns, and the option to carry four fixed .50 guns in packages on the sides of the forward fuselage. The ventral gun would be eliminated on the final B-26F and G models. The extra weight and space required for this defensive armament did force a reduction in offensive armament loads. The B-26F and B-26G also saw the elimination of the aft bomb bay, which reduced the theoretical, but rarely fully used, bomb capacity from 5200 lb to 4000 lb.
The Martin bomber proved to be less versatile than the B-25. It did have its strengths. It was fast and rugged, it had good defensive armament and a roomy bomb-bay. But the B-25 had the qualities more desired of a tactical bomber based close to the front line, because it could operate from shorter airfields, was easier to maintain, and had a better availability rate. The Mitchell also proved more adaptable to low-level attack missions. (A solid nose with two 37-mm cannon was developed for the B-26, but never adopted.) Gradually but entirely, the B-25 replaced the B-26 in the Pacific. An evaluation of combat experience in the the Mediterranean theatre indicated that the B-26 had nearly twice the loss rate of the B-25 and a lower operational availability. By mid-1943 it was planned to phase out production of the B-26 to free capacity to build more B-29s, and to complete many Marauders as trainers or target-towing aircraft without combat equipment.
However, at this time a mission profile was found that suited the B-26 better. Pioneered by the RAF, level bombing in tight formation with a fighter escort, from medium altitudes of 2500 to 4000 m, proved an effective tactic when the B-26 was equipped with the Norden bomb sight. Targets tended to be of tactical nature, such as airfields, railways, and V-1 launch sites. During the invasion of Normandy on 6 June 1944, Marauder units were tasked with precision attacks on German coastal batteries. Incidentally, RAF medium bombers operating on this day were a mix of Mitchells and Mosquitoes, while the US 9th Air Force flew the B-26.
Finding a new role for the B-26 allowed production to continue with the B-26F and B-26G, although one factory was still switched over to B-29 production. (Martin would build 531 of those.) Most B-26F and B-26G bombers would be sent to existing American units in Europe and the Mediterranean, as well as the RAF and the renewed French air force.
The B-26 was a successful bomber, but it is notable that the B-25 and B-26, although they were built to the same requirement and were somewhat similar in configuration and armament, were successful in different mission profiles. The Marauder took advantage of the latest technology, but was a more demanding aircraft to operate, also in terms of pilot training, maintenance, and base infrastructure. Maybe Martin should have taken the logical step of adding more high-lift devices to the B-26’s wing, which could have improved take-off and landing performance at the cost of higher complexity. (Lockheed delivered large numbers of twin-engined aircraft equipped with Fowler flaps, so this was not impossible to do.) As it was the problems with the B-26 contributed to the souring of the relationship between Martin and the air force, and nearly lead to the type’s demise.
Patrol Bombers
There is another family of American combat aircraft which are relatively easy to overlook but deserve mentioning in this context, even though they were not, realistically, very successful medium bombers.
Before WWII, Lockheed had produced a family of small, fast twin-engined airliners. The Lockheed 10 Electra for ten passengers and a crew of two was followed by the Lockheed 12 Electra Junior for six passengers, the Lockheed 14 Super Electra for twelve passengers, and the Lockheed 18 Lodestar for 17 passengers. They were not the most economic of aircraft but they were resolutely modern and offered high performance. Amelia Earhart chose a Lockheed 10E for her flight around the world; Howard Hughes completed his in a Lockheed 14. More infamously, Neville Chamberlain was flown to Germany to negotiate with Hitler in a Lockheed 14. Small numbers of Lockheed transports were also sold to foreign air forces, including 30 Lockheed 14s for Japan and a license to build more.
It was Lockheed’s own initiative to offer a military version of the Lockheed 14 to the British Purchasing commission in late 1938. Its order books were almost empty, so this was not an opportunity that it could afford to let pass. The British were sufficiently interested to invite the design team to travel to London. A few days of intensive design work and negotiation in April 1938 paid off for Lockheed, which received a $25 million order which enabled it to expand its operations and triple the size of its workforce. Under the terms of the contract, Lockheed was required to deliver 200 aircraft, but this was later amended to 250 if Lockheed could deliver them in 1939. The first Lockheed 214, with the British designation Hudson, flew on 10 December 1938. When war broke out in September 1939, the RAF had already taken delivery of 250 Hudsons. Production would finally run to 2914, of which about two-thirds went to Britain. Smaller numbers were operated by Canada, Australia, the USA, Brazil and China.
The Hudson retained some of its airliner characteristics, notably a row of windows along the sides, but it was much re-arranged internally to make room for its military equipment, in particular a bomb bay underneath the main cabin. The British provided a rather tall Boulton-Paul gun turret which was installed close to the twin tail fins. The nose was redesigned with transparent panels for the navigator, also the bomb aimer, and two fixed 7.7mm Browning machine guns were installed. Ventral and beam guns were added on some later aircraft. The Hudson didn’t have strong defensive armament, but nevertheless one was the first RAF aircraft to shoot down a German aircraft, a Do 18 flying boat over the North Sea.
The Hudson had been intended for Coastal Command, as a naval bomber and reconnaissance aircraft to replace the Avro Anson. Though the type had a decent performance, its weak defensive armament and modest bomb load (1600 lb maximum but more commonly 1000 lb) made it rather ineffective as a bomber. It was nevertheless used for attacks on enemy shipping in port, and helped the complete the numbers on the 1000-aircraft raids of Bomber Command. But primarily it was a maritime patrol aircraft, and gave excellent service as such. The Hudson was also used to transport agents and materiel across the channel to the French resistance, which speaks for a decent short-field performance. (Though Capt. Eric Brown disliked the trim changes and aerodynamic instability that came with the deployment of the large Fowler flaps.)
Back in 1940 the British Purchasing Commission had also shown interest in a larger and more powerful aircraft, based on the Lockheed 18 Lodestar. The Hudson had, depending on the model, been powered by the Wright R-1820 (these aircraft being the A-29 or PBO-1 in US nomenclature) or Pratt&Whitney R-1830 (A-28), but the Ventura was initially fitted with the Pratt&Whitney R-2800 and later with Wright R-2600. The addition of a ventral gunnery position with two .303 machine guns, and the relocation of the dorsal turret to a position not quite as close to the tail, where visually distinguishing features of the Ventura, which otherwise looked very similar to the Hudson. Maximum bomb load was increased to 2500 lb (1100 kg) which could make the Ventura a more useful bomber. Conversely, it had thirstier engines but the fuel capacity was not increased, making it less useful as a naval reconnaissance aircraft. And when the type entered service in 1942, the RAF quickly discovered to the Ventura Mk.I was not a good medium bomber. Accordingly, the Ventura was shifted to the patrol bomber role, but became largely unwanted in the RAF inventory.
Although the USAAF took some as B-34 (with R-2800 engines) or B-37 (with R-2600 engines) bombers and used them for training, most of the production run of about 1600 were handed over to the US Navy, who adopted the type as the PV-1 Ventura patrol aircraft. Aware of the type’s deficiencies, the Navy engaged Lockheed’s Vega division to redesign the Ventura to perform better in its expected role. The new aircraft had larger wings, more than twice the fuel capacity of the original design, a redesigned tail, radar, and an enlarged bomb-bay. It was powered by 2000hp R-2800-31 engines and fitted with five fixed .50 machine guns in the nose, a pair in a Martin 250 dorsal turret, and one in the ventral position. On 30 June 1943, the US Navy placed on order for 500 of the PV-2 Harpoon. The prototype flew before the end of the year, but problems with the structural integrity of the new wing required additional major redesign. The first combat missions of the PV-2 were thus delayed until March 1945, and the production run of the Harpoon stopped at the 535th aircraft. In its developed form the PV-2 was well liked, however, and continued in service into the 1950s, with small numbers being distributed to various allies. It would be overshadowed by the highly successful P2V Neptune.
Visions of the Future
The B-25, B-26 and to some extent the A-20 provided the US Army Air Force with versatile and effective medium bombers, but none of these aircraft offered exceptional performance. Plans had been made for a number of aircraft that would, however. In the end none of them were required for WW2.
As mentioned, there was early interest in the possibility of a high-altitude twin-engined bomber, and the initial requirement for the A-20 had even specified turbocharged engines, but this was soon dropped. This was not the only venture in this direction, as in December 1936 North American had flown its NA-21 high-altitude bomber, with turbocharged R-2180 engines. The bulky and somewhat awkward NA-21, renamed NA-39 in modified form, would not enter service, although the Air Corps purchased the sole prototype as the XB-21 in early 1939. In August 1939 a new XC-214 specification was written for a high-altitude medium bomber. The concept brought together turbocharged engines, a pressure cabin, and remote-controlled armament. This was the same set of technologies that would be incorporated into the B-29 heavy bomber, but to be merged into a much smaller aircraft. At the time, the B-25 and B-26 had not even flown yet, but their future manufacturers were natural candidates for the XC-214 program. Martin offered its model 182, to which the Air Corps assigned the designation XB-27, but it was never built. A prototype of North American’s NA-63 was ordered as the XB-28, but because other programmes received a higher priority, it would not fly until 26 April 1942.
The XB-28 didn’t have much of a family resemblance to the B-25. It had a single tail fin instead of twin fins, a round fuselage cross-section instead of a rectangular one, and generally perhaps a somewhat Soviet look. It was powered by turbocharged R-2800-27 engines and remote-controlled gun turrets were installed in dorsal, ventral, and tail positions. (The first prototype had North American designed turrets, the second turrets designed by General Electric.) It had a crew of five and a maximum take-off weight of 17 tons. As impressive as the aircraft was, there was no longer an operational need for it. When it came to high-altitude bombing, a B-29 would do the job very well, and when it came to low-level bombing, the complexity of the XB-28 was unwanted. A second prototype was completed as a reconnaissance aircraft, and flew on 24 April 1943, but was soon lost during testing.
Another remarkable design was initiated by Douglas in early 1943. To improve aerodynamic efficiency, in particular to increase range, the Douglas model 459 had its two liquid-cooled Allison V-1710 V-12 engines buried in the fuselage, behind the cockpit. They would drive three-bladed contra-rotating pusher propellers, located aft of a cruciform tail. It was a high-risk arrangement, as this required long extension shafts, but it was aerodynamically efficient and left room for a bomb bay in the lower fuselage. Douglas received a contract for the development of the XA-42, later renamed XB-42, in June 1943. The first flight was in May 1944, an impressive pace of development. But when the war ended, vibration problems remained to be resolved, and even a small pre-series was not proceeded with. The development of the jet-powered XB-43 derivative was already ongoing in parallel, but this would not fly until May 1946, and in the end would be cancelled in favour of the more conventional North American B-45.
The aircraft that would actually replace the A-20, B-25 and B-26 then, was not revolutionary. According to Ed Heinemann, the Douglas A-26 was designed in response to the desire of the Air Corps to have a 75-mm cannon in the nose of the A-20, an option that he rejected because he felt that the aircraft was too small to carry this burden. Instead a new aircraft was designed with a wider fuselage, and Douglas received a contract for three prototypes in June 1941, followed by a first order for 500 production aircraft in October.
The A-26 was fairly conventional in layout, with a laminar flow wing and two R-2800 engines. The wider fuselage allowed the pilot and navigator/gunner to sit side-by-side, while a third crew member defended the rear with remotely-controlled dorsal and ventral turrets. Douglas saved weight by using new, stronger aluminium allow, and aimed for the cleanest possible surface finish, though it may not have met the high standard of smoothness required to truly achieve laminar flow over the wing. Douglas had proposed versions with different noses. The XA-26 had a glass bomber nose with seating for a bombardier, the XA-26A was a night intruder with four 20-mm cannon in a belly pack, and the XA-26B carried a 75-mm cannon in the nose. In the end the 75-mm cannon was never used operationally by the A-26. The two production versions were the A-26B, which had a nose with six .50 machine guns, and the A-26C, with the transparent bomber nose. The Invader was well suited for low-level attack operations, but both the RAF and the 5th Air Force in the Pacific found fault with the view from the cockpit, which was only good straight ahead.
The engineering goals were ambitious enough to cause development and production delays, and although the XA-26 flew in July 1942, the first production aircraft was not delivered until September 1943. Production continued at a slow pace, and the first ones arrived in the Pacific and in Europe in May and June 1944, respectively. Although it proved to be an excellent aircraft, the end of the war meant that it was never produced in the numbers planned; instead production halted in August 1945 with 2,529 aircraft completed. This was enough to equip the light bomber squadrons of the post-war air force, and the A-26 quickly displaced the A-20, B-25 and B-26. In 1948 this resulted in the Douglas aircraft being redesignated B-26.
The Invader achieved post-war notoriety in the 1950s and 1960s, after the type was gradually withdrawn from USAAF service and became available for covert operations and export to friendly states, with aircraft being distributed to a surprising number of countries and participating in numerous conflicts. It was obsolescent, but still an effective attack aircraft in areas were there was little to fear from enemy opposition in the air. Most remarkably, the type was adopted by the CIA for covert operations, including an intervention in Indonesia in 1958, the “Bay of Pigs” attempt to overthrow the Cuban government in 1961, operations in Laos in 1961, and in Congo in 1964. The type also served in the Vietnam war, both with the French air force and with USAF units.
Because the B-26s were running out of fatigue life, the USAF contracted with the company On Mark for the development of the final variant, the B-26K, in 1963. On Mark had experience reconditioning surplus A-26s as civilian aircraft, and it was to completely overhaul 40 aircraft. This included a rebuild of wings and fuselage as well as the fitting of new R-2800-52W engines and new propellers. The B-26K had eight wing pylons for external loads, wing tip tanks, and a new nose with eight .50 machine guns (a transparent nose was still a possible alternative fit). The new B-26K was faster, had more than double the tactical combat radius, could carry a heavier weapons load, and had updated cockpit equipment, but was still fundamentally an Invader. Handling characteristics had suffered, for the B-26K was less stable and had a higher minimal controllable speed on one engine. In June 1964, twenty years after the A-26 entered service, the B-26K was again delivered to the USAF. In May 1966 the B-26K was officially redesignated the A-26A, because it had been agreed to base a number in Northern Thailand, and because the A designation was associated with a less offensive role than the B, it was more politically acceptable. Operations ended in 1969 because of a dwindling number of available aircraft, maintenance issues, and the availability of the new “gunships” such as the AC-119 and AC-130. Today a 25-year service life is not that remarkable for a combat aircraft, but for a type designed in the 1940s it was quite an achievement.
The American Experience
The United States built large numbers of medium bombers for its own use as well as for export or lend-lease to its allies. They had a global reach. The heavy bomber force, though very large, was mostly concentrated in hotspots where the necessary infrastructure could be built (a very major undertaking) and strategic targets were in range. Medium bombers went everywhere, often to regions were ruggedness and ease of maintenance were essential.
The availability of different noses for the A-20, B-25 and A-26 conferred them an adaptability to starkly different operating environments. In theatres that were “dense”, with a continuous high risk of exposure to enemy fighters and AAA, level bombing from medium altitude with a fighter escort was often the preferred tactic. Such conditions prevailed in Europe. But in theatres that were “sparse”, with enemy opposition possibly strong but concentrated in a few distant locations, low-level strafing attacks could be more effective. These conditions were found in the island-hopping campaigns of the Pacific. With changes in equipment, American medium bombers demonstrated their value in these very different conditions. Probably no other nation was as successful in ensuring operational flexibility.
That said, the USA also successfully developed both heavily armed long-range fighter-bombers, and increasingly sophisticated heavy bombers. In the squeeze between these categories, the role of the medium bomber diminished post-war, though there still was a place for aircraft such as the A-26, B-45 and B-57.
Flying Guns 