In a common retelling of the air war component of World War II, the heavy bomber is the decisive weapon, and the users of medium bombers are at best portrayed as people eagerly awaiting an upgrade of their equipment. And at worst, as people who were not smart enough to get heavy bombers. The latter criticism is most frequently made of the German Luftwaffe, while other air forces that did not invest a lot of effort into four engined heavy bombers, notably those of France, Italy, Japan and the USSR, are mainly excused on the grounds of poverty.
Simplistic accounts of the Battle of Britain will often state that the Germans lost the fight because they did not have long-range heavy bombers. This explanation of course overlooks that many vital targets in Britain were within range of the Luftwaffe’s existing force but were not effectively attacked, and rather too easily skips over other serious structural problems of the German air force. By extension, the Luftwaffe is routinely condemned for having made a strategic error by not investing in heavy bombers (apart from small numbers of the flawed He 177), without considering how these would have fit in German strategy, or where the necessary materials and fuel could have come from. In contrast, the RAF is portrayed as being of the right mind, but struggling until the “heavies” were introduced, the Stirling, the Halifax and especially the Lancaster. Again, that is a simplistic explanation that overlooks both the array of technological evolutions that made the strategic bomber offensive viable, and the wide range of other tasks undertaken by the RAF.
After the war ended, the US and British air forces both had a vested interest in emphasising the importance of the strategic bombing offensive, because this was perceived as their independent contribution to victory. It was to their institutional advantage to emphasise it, and the appearance of nuclear weapons made it even more important. Thus it became the main topic of the debate on the role of the air forces during WW2, and the topic of strategic bombing surveys that fed that debate. It helped that the offensives of the strategic bomber force such as the “Big Week” (Operation Argument), the raid on Ploiesti (Operation Tidal Wave), and even the bombing of Dresden were relatively self-contained stories, and thus simpler in the retelling than the complex air-land cooperation that underpinned tactical missions.
In this discourse, apparently the use of large numbers of medium bombers by the USAAF and RAF was no hindrance for portraying these as second-tier equipment. They were something the big players still had in common with less fortunate air forces such as the Luftwaffe and the Soviet VVS, but that was an embarrassing coincidence. History was often written around the heavy bomber.
Origins
The origins of the distinction between the “medium bomber” and the “heavy bomber” can be sought within the first large air war, as a growing and developing part of the First World War. When war began in 1914 the major powers already had small air forces. The primary task assigned to these was reconnaissance, as the performance of most aircraft in service did not permit the carrying of significant bomb loads. By 1916 this had changed, and large multi-engined bombers took their place besides the smaller single-engined types.
As the warring nations planned the development of their air forces, each adopted its own tactical, operational and strategic ideas and the associated terminology. It can therefore be a bit misleading to divide WWI bombers into “light”, “medium”, and “heavy” types, as such a general terminology did not yet exist. Nevertheless there are signs of a developing specialisation and a distinction between the general-purpose bomber and the “heavy”. The long range and large bomb load of the latter permitted its use in a strategic role.
France
France had, due to its leading position in aviation technology at the outbreak of war and the size of its forces, an early opportunity to set the stage. However, as the Western Front ran through French territory, it was natural for them to invest primarily in tactical aircraft. French development of single-engined bombers was significant and successful, ultimately leading to the Bréguet 14, one of the best light bombers available at the end of the war. Its investment in larger multi-engined aircraft was much more modest. Yet when Marshal Pétain (yes, that Pétain) proposed ambitious plans for a force of 6,000 aircraft in 1918, this included both medium bombers and strategic heavy bombers.
The medium bomber was defined as a “Bn2” type, that is a medium night bomber, while faster types would operate during daylight. Significantly, the Bréguet 16 was a derivative of the Bréguet 14 with a longer wing span to enable it to carry a heavier load of fuel and bombs. The single engine, a 300 hp Renault 12Fe, was the same as found on some (by no means all) Bréguet 14s. About 200 were built, but the type did not see service during WWI. Another common type used for the “medium” night bomber role was the Voisin 8 (and its derivatives), a single-engined pusher type that was obsolescent by the end of the war.
The heavy bomber, in French thinking, was a multi-engined night bomber, described as a “BN2”, “BN3” or “BN4” depending on the number of members of the crew. This was a more ambitious role. The chosen type was the Caudron C.23, a twin-engined three-seat type with the range to stage attacks on Berlin. It had a well-streamlined if somewhat bulky fuselage, and long-span biplane wings. Defensive guns were installed in the nose and in a dorsal mount. Although a 1000 were ordered and 54 were delivered by the end of the war, no C.23 saw action. It would only enter service after the end of the war, when it was discovered to be seriously underpowered by its two 260 hp Salmson Cu-9Z engines, and soon replaced by the Farman F.60.
The most significant heavy bomber that was in service with the French during WWI itself was an Italian design: The Caproni 3, a twin-boom aircraft with three engines, two installed as tractor engines at the front of the tail booms, the third as a pusher engine in the central nacelle. French attempts to build the type with French engines were relatively unsuccessful, and it appears that the operational aircraft were mostly imported from Italy, while Italian units also operated the type on the Western Front. The Capronis were used from February 1916 mainly to attack communication centers and transport behind the enemy front lines, although attacks were also staged on German factories, notably in Ludwigshafen. Arguably, they were being used in what would later be considered a medium bomber role, despite their designation.
Germany
Superficially, the situation appears more clear-cut in Germany, where a distinction developed between the Grosskampfflugzeug and the Riesenkampfflugzeug, between the “large bomber” and the “giant bomber”. However, the key distinction between the “G” and “R” types was not defined by their operational role, at least not directly, but instead was a basic technical one: For the “R” types it was a requirement that the engines were accessible, and thus maintainable, in flight. This was a reasonable precaution for long-range flights using the technology of WWI, and the R types were intended as long-range strategic bombers. In operations against England, the R types would be able to complete 93% of their missions, as opposed to 76% for the Gotha G types, which speaks for the rationality of the requirement. However, it was a requirement that required a large aircraft. Designers could opt for substantial engine nacelles, or try to install the engines in the fuselage and drive the propellers through gearing and shafts.
Many of the Riesenkampfflugzeug designs were excessively complex, lumbering giants of dubious usefulness. The most successful R types were produced by Zeppelin Staaken. Interestingly, from 1915 the firm experimented with 160 hp Mercedes D.III engines installed in pairs with a gearbox to drive a single propeller; thus the Zeppelin Staaken VGO.III had six engines but three propellers. A concept that would be repeated by the Heinkel He 177 in the next world war! This arrangement was successfully employed on the Staaken R.IV but the R.VI, the most numerous of the Staaken series, instead had four engines (260 hp Mercedes D.IVa or 245 hp Maybach Mb.IVa) installed in tandem push-pull nacelles between the wings. With a loaded weight of 11,460 kg and a wing span of 42.2 m, the R.VI was a successful heavy bomber.
From the night of 28 September 1917 onwards, R planes did fly strategic missions against England, at night. But significantly, only two R planes operated that night, and 25 G types. This was an unusually large raid, but the pattern persisted in which R planes were employed only in small numbers. The peak effort would be six R planes of Rfa 501 in the night of 7 March 1918. As the total number of R planes produced by the German industry in WWI was only about 60 of all types combined, most of the bombing offensive was flown by AEG, Gotha and Friedrichshafen G types. Not that these were employed in huge numbers: When the war ended there were 244 G types at the front. Still, they were easier to build and easier to maintain. The smaller aircraft ended up being responsible for three-quarters of the bombs dropped on UK targets.
These were twin-engined biplanes and the major production versions were powered by the 260 hp Mercedes D.IVa engine. Performance differences resulted in the relatively short-ranged AEG G.IV being assigned to tactical missions, but the type was appreciated for its robustness and good handling, and remained in service until the end of the war. The Gotha G.III, G.IV and G.V were the mainstay of the long-range bombing offensive of 1917, with the most important production version being the Gotha G.IV. The Friedrichshafen G.III appeared relatively late in the war and as it was capable of carrying a substantially heavier bomb load than the Gotha, it became the preferred type. All three had a defensive armament consisting of a gunner in the nose and another in a dorsal position, but the Gotha G.III pioneered an arrangement in which the bottom of the aft fuselage was an open “tunnel”, permitting the dorsal gunner to defend the lower aft sector as well.
A directive of 25 April 1917 established several Kampfgeschwader der Obersten Heeresleitung, bomber units assigned to the Army High Command for strategic use. From March of that year, Gotha G.IV bombers were gathered in Flanders for a bombing offensive against England. Although initial operations were flown in daylight, from September of the year stiff opposition forced a switch to night bombing.
The G types were not exclusively used for strategic attacks however, and indeed from May 1918 onwards the crisis at the front results in a shift away from attacks on London and Paris, to targets of more immediate tactical value. Railway stations in particular were high priority targets to disrupt enemy supply lines and potentially stall offensives.
German G types were also supplied to Austria-Hungary. The dual monarchy had a small aviation industry and the only viable multi-engined bomber it produced was the Aviatik 20.23, flown in early 1917. Plans to put a development of this into production as the Aviatik G.III were cut short because of the end of the war. Austria-Hungary fell back on using German designs. In 1915 encouraging results were achieved by the twin-engined Brandenburg 05.05, a German design built to Austrian specifications. But the production Brandenburg G.I was disliked for its flight characteristics, structural weakness, and inability to maintain flight on one engine. After a single mission, they languished in storage. In 1917 and 1918 the Germans allowed their ally to purchase a small number of the Gotha G.IV bombers and later also the Friedrichshafen G.IIIa, but only the G.IVs arrived in time to see service, fitted with 230 hp Hiero engines that proved troublesome. It is possible that the geography of the Austrian fronts, especially along its Alpine frontier with Italy, put more stress on aircraft than operations from the Low Countries.
Given that a Gotha G.IV had only a third the loaded weight of the Staaken R.VI, it seems fair to consider the G types medium bombers. Also in other ways the G series could be regarded as ancestral to the medium bombers that Germany used in WWI: Their twin-engined configuration, the basic layout of their defensive armament, and their operational use (both strategic and tactical) prefigure types such as the He 111 and Ju 88. Another interesting parallel is that between the small production of Riesenflugzeuge during WWI, and the similarly small production of heavy bombers by Germany during WWII. We probably should not exaggerate the connections, because the treaty of Versailles banned Germany from possessing combat aircraft, enforcing a hard stop in 1919.
Britain
In Britain, the growing interest in strategic bombing was a driving factor behind the creation of the RAF on 1 April 1918 as an independent air force, taking over from the earlier RFC (Royal Flying Corps) and RNAS (Royal Naval Air Service). In both forces, the primary bomber types had been single-engined “light” types, but there were exceptions. The RNAS took the lead in the development of long-range bombers, maybe because it was lead by naval officers whose instinct was to look beyond the frontlines on land.
One of the first was simply known as the Short Bomber. The RNAS wanted an aircraft suitable to attack ships in port and shore installations. The Short Bomber was an interim type, a single-engined but much enlarged derivative of the Short 184. As well as seeing some tactical use, it was employed in 1916 and 1917 to attack German submarine bases at Zeebrugge on the Belgian coast. However, by then the introduction of multi-engined types had already made it obsolescent.
The RNAS was also responsible for the development of the O/100 by Handley Page. Commonly considered the first RAF heavy bomber, the O/100 was a twin-engined biplane with a crew of four. Targets continued to be primarily naval however, including German submarine bases and warships. This changed as a result of German bombing attacks on England and the creation of the RAF, and the improved O/400 was assigned as a night bomber to the Independent Force to perform strategic attacks. Meanwhile the RAF awaited the delivery of the much larger V/1500, which would have mounted an attack on Berlin if bad weather and the Armistice had not prevented it. The V/1500 was definitely a heavy bomber, with four 375 hp Rolls-Royce Eagle VIII engines, installed as in tandem push-pull nacelles between the wings. (In this it was similar to the Zeppelin Staaken R.VI.) In the near future, the RAF awaited the twin-engined Vickers Vimy, which was also considered a heavy bomber, capable of flying raids on Berlin, though it did not enter service before the end of the war.
Meanwhile, a type emerged that can be considered a “medium”: The Airco D.H.10 Amiens. The Amiens was a twin-engined day bomber. Its direct ancestor was the D.H.3 of early 1916, a compact aircraft powered by two 120 hp Beardmore engines, which made a good impression. The D.H.3 was first ordered and then cancelled, in murky circumstances but probably because in part because it was a private venture not built to a formal War Office specification, and in part because of Army skepticism about multi-engined bombers. (The much larger Handley Page O/100 was developed in the same time frame, but crucially, the customer for it was the Admiralty.) In July 1917 a formal specification for a fast two-seat day bomber was ready and officialdom even suggested that a D.H.3 development with 200 hp engines would be suitable. After considerable redesign it took the 400 hp Liberty 12 to meet the specifications, in the Amiens Mk.III. As it was July 1918 before the prototype Mk.III flew and there were delays in the delivery of the Liberty engine, the WWI combat service of the Amiens Mk.III would be limited to one mission by one aircraft!
Only 258 were completed and their service life in the RAF was relatively short, but they did see action on India’s North-Western Frontier. While the Short Bomber, O/100, O/400, and V/1500 all were slower than 100 mph (and the Vimy only just exceeded that speed), the Amiens was good for 131 mph at sea level and 124 mph at 10,000 ft. On the other hand, its bomb load was modest, only 920 lb, or 12% of what a V/1500 could lift. Though not a very significant aircraft in RAF history, it was a precursor of later fast medium bombers. But the terminology of “medium” and “heavy” bombers only entered official RAF aircraft specifications post-war.
Russia
When Bill Gunston created The Osprey Encyclopedia of Russian Aircraft 1875-1995 in 1995, he created a book of 560 pages, but a mere 15 of those covered the aircraft built before the Russian Revolution. This reflected the general backwardness of the industry of the Russian Empire, which could in no way compete with the other major powers. But that did not stop the RBVZ firm, with its chief designer Igor Sikorsky, from creating some of the largest combat aircraft in the world.
In March 1913 Sikorsky himself made the first flight of the aircraft that would become known as the Russkkii Vityaz, the “Russian Knight”, a giant with a wing span of 27 meters. Several alterations resulted in a definitive configuration with four 100 hp Argus engines and a gross weight of 4200 kg. It led to a series of bombers and reconnaissance aircraft known as Ilya Muromets, after a hero of Sikorsky’s native city of Kiev. There were numerous variations in configuration, engines and armament, leading to aircraft that were considerably bigger, heavier and more powerful than their ancestor, with loaded weights over 7 tons in the final version. They were heavy bombers by any standard, operated successfully by the “Squadron of Flying Ships”.
For this same unit, Vassili Khioni designed a new aircraft that was more a medium bomber, known as the VKh Anasalya. More modern conceptually than the heavy bomber, it was a twin-fuselage biplane. The two Salmson radial engines were in the front of the fuselages which were attached to the lower wing, while a nacelle for a gunner was added centrally on the upper wing. Although 50 were ordered, but the Revolution prevented any deliveries.
Italy
Bomber development in Italy was dominated by the aircraft designed by Gianni Caproni, whose influence was such that Jane’s dubbed him “the Apostle of big planes.” He embarked on two main lines of development, both characterised by a twin-boom layout with an engine in the front of each boom and a central nacelle that had a pusher engine. The Ca.1, Ca.2, Ca.3 and Ca.5 were a family of biplanes that saw gradual development. The Ca.4 was a much bigger aircraft of triplane configuration, and one of the largest bombers of the period.
The Ca.1 made its first flight in October 1914 and production of a first batch was initiated in March 1915, with three 100 hp engines. They were followed first by the slightly more powerful Ca.2 and then by the Ca.3 with three 150 hp Isotta-Fraschini V.4B engines, which was the more mature form of the type. The Ca.3 was used against targets behind the enemy front lines, but when disaster struck the Italian army in late 1917, also against enemy troop concentrations.
The Ca.5 retained the general layout of the earlier types but was redesigned in detail, with longer span wings, an aerodynamically cleaned up fuselage, and 200 hp Fiat A-12 engines. The A-12 unfortunately turned out to be unreliable. The Ca.5 was also ordered into production in the US, with 450hp Liberty engines, after a comparison showed its performance to be superior to that of the British 0/400, which admittedly could carry a heavier bomb load.
The heavy Ca.4 triplane of 1917 was less successful. The problem was not the type’s performance, which was good, but its structural complexity, which made it a challenge to build, and less than 50 were completed.
Thus the primary Caproni types were the Ca.3 and Ca.5, which can be thought of as medium bombers by comparison with the huge Ca.4, though they were considered heavy bombers by contemporaries. Though now largely forgotten, they were famous in their time, also by their association with men like Gabriele D’Annunzio (famous poet and proto-fascist) and Fiorello LaGuardia (later a long-serving mayor of New York city). Both France and the USA adopted the Caproni biplanes as heavy bombers, for lack of satisfactory national alternatives. Interestingly, several Italian medium bombers of WWII would persist with a three-engined configuration.
Summary
There are important difficulties with defining the medium bomber in the context of World War I. When the most combat combat aircraft type was a single-engined biplane, including the most common types of bomber (as was emphatically the case in France), any multi-engined bomber could be considered a heavy bomber. Yet both in Germany and in Italy, and to a lesser extent in Britain, there was interest in the development of the strategic “heavy bomber” type, represented by aircraft such as the R types in Germany, the Ca.4 in Italy, and the V/1500 in Britain. By comparison, smaller multi-engined bombers could be considered “medium bombers”, even if this terminology was not used at the time.
Intended and actual use must be considered at least as significant as considerations of size and weight. The largest bombers justified their existence by a putative strategic role, in which their range and large bomb capacity offset their cost and relative inflexibility. The cost of producing and operating them limited production to small numbers. The twin-engined biplane types were large enough to offer good carrying capacity but small enough to be affordable,. They proved to be a flexible weapon suitable for long-range attacks on enemy cities and industry, but also supply and transportation targets behind the enemy front line, and tactical targets if the need arose.
Table
| Caudron C.23 | Gotha G.IV | Airco D.H.10 Amiens Mk.IIIa | Caproni Ca.3 (Ca.33) | |
|---|---|---|---|---|
| Crew | 3 | 3 | 3 | |
| Engines | 2 x 260hp Salmson Cu-9Z | 2 x 260 hp Mercedes D IVa | 2 x 400 hp Liberty 12 | 3 x 150hp Isotta-Fraschini V.4B |
| Wing Span (m) | 24.47 | 23.7 | 19.96 | 22.74 |
| Wing Area (m2) | 106 | 89.5 | 77.8 | 95.64 |
| Length (m) | 12.98 | 12.2 | 12.07 | 11.05 |
| Height (m) | 3.45 | 3,9 | 4.42 | |
| Empty Weight (kg) | 2341 | 2413 | 2608 | 2300 |
| Loaded Weight (kg) | 4170 | 3648 | 4110 | 3800 |
| Max Speed (km/h) | 144 | 135 | 211 | 140 |
| Max Range (km) | 700 | |||
| Ceiling (m) | 4500 | 5000 | 5800 | 4800 |
| Armament | 2 machine guns | 2 or 3 Parabellum machine guns | 2 x .303 Lewis guns | |
| 600 kg bombs | 417 kg bombs |
Flying Guns 