Cannon with a calibre of over 37mm have never become popular as aircraft weapons, despite many attempts to adopt them, both for air-to-air combat and for air-to-surface combat. Currently they are very rare. The main attraction of such weapons has always been the possibility of a single-shot kill of a large bomber, a small ship, or a tank. The disadvantages of these weapons are excessive weight, limited ammunition supply and low rate of fire. Recoil is also a significant problem, but in theory this is one that can be avoided by the development of recoilless guns. However, no recoilless gun has ever been really successful.

Early Experiments

Among the earliest cannon fired from aircraft were the Hotchkiss weapons of calibre 37mm and 47mm, weapons constructed using parts of the automatic cannon of the period. The 37mm was produced in significant numbers, and available in version with a long and a short barrel. A few hundred French aircraft were equipped with them during WWII; mostly on the Breguet 5 and the Voisin 4, both single-engined aircraft of pusher configuration. Because they were loaded manually, the rate of fire was very low; and they were not very effective in air-to-air combat. They were useful as ground attack weapons, however.

The French identified another possible use for large cannon: As anti-submarine weapons. During WWI aircraft were slow enough that a spotted submarine might be able to dive before the aircraft could drop bombs on it. A cannon extended the reach of the anti-submarine aircraft. This culminated in the concept of a “high seas flying boat”, for which a specification was written in 1918. This would be an aircraft with a crew of four, an eight-hours endurance, and an armament including two machineguns, 120kg of bombs, and a 75mm cannon with 30 rounds! The aircraft designed to this specification never entered service, but the Tellier T.7 did serve as testbed for the 75mm cannon between 1920 and 1922.

In Britain too, experiments were conducted with a number of large-calibre weapons, mainly for use against balloons and airships, although they were also used for ground attack. Vickers delivered the Vickers 1½-pounder and 1-pounder guns, the 1.59in Vickers Crayford, also known as the “Rocket Gun” because its incendiary projectile left a trail of sparks, and the Vickers 1-inch gun. The automatic 1-pounder cannon, basically a much enlarged Maxim machinegun, was the most successful, but nevertheless remained rare. Its recoil was the largest disadvantage.

More common than the Vickers guns was a weapon developed by the Coventry Ordnance Works, the 37mm 1½-pounder COW gun. The COW gun was automatic, very light for a weapon of this calibre, and had a good ballistic performance. But nevertheless it was a bulky weapon, and it saw little service. During the the interbellum it was carried by a few large aircraft, and a handful of fighters were designed round it, but none of these installations was adopted by the armed forces. The weapon seems to have seen its only actual service on the ground, as anti-aircraft gun…

Cleland Davis designed the recoilless guns named after him. They were simple weapons, basically a barrel with two open ends; the recoil was compensated by firing a lead shot rearwards. The loading procedure required the barrel to be made in fore and aft pieces, with a breech join at the center, so that the round could be inserted manually. Leaks at this joint were a serious design problem. Three versions were produced, a 2-pounder, a 6-pounder and a 12-pounder. Its service life, mainly as anti-Zeppelin weapon, was short. The lead shot fired from the rear, and the rearwards blast, made this weapon highly inconvenient to install in the fragile WWI aircraft. It was installed on some aircraft, but generated little enthusiasm among the crews who had to use it.

One of the first aircraft designed for a really large cannon was the Admiralty Type 1000, also known as the AD.1. The Admiralty’s concern was the German fleet, and it planned three versions: A bomber, a torpedo bomber capable of carrying an 18 inch torpedo, and a gun machine armed with a recoilless Davis 12-pounder gun. The latter would be used to lob shells at small warships from a safe distance. Development of the giant seaplane began in 1915, and it was completed and flown in the summer of 1916. Concerns about the rearwards blast of the Davis gun caused to design to be changed for a 12-pounder Naval Landing Gun, a conventional cannon, that would be installed on a mount allowing 49 degrees elevation and 38 degress depression. In the end, no gun was ever installed in the AD.1. And probably much for the better.

Soviet Recoilless Guns

The recoilless Davis guns inspired the development by B.S. Stechkin and L.V. Kurchyevskii of a series of similar weapons in the USSR. Between 1930 and 1936, when he was arrested and disappeared, Kurchyevskii developed a series of guns, that were installed in experimental and even production fighters. The design of all these aircraft was influenced by the rearwards firing of a compensating mass. Either the guns had to be in the wings, or the barrel had to be extended to the extreme tail of the aircraft.

Project Z, also called TsKB-7, was a small low-wing monoplane fighter developed by Grigorovich. He used parts of the I-5 biplane fighter to speed development. The aircraft had a recoilless 76.2mm DRP under each wing, and a single 7.62mm PV-1 machinegun in the fuselage to assist in aiming. About 50 production aircraft, called I-Z, were built. But because they suffered from handling problems, and the DRP guns were single-shot weapons, they were mostly used for further development work. Grigorovich followed with the IP-1, a refined aircraft armed with APK-4 guns at the wingtips. These could fire five rounds. Although the IP-1 entered production, it was without the recoilless guns: The 20mm ShVAK was preferred. The IP-4, with four 45mm APK-11guns, remained experimental. Later Grigorovich fighters still had heavy armament, but significantly, stuck with multiple conventional 20mm cannon.

Meanwhile, work had also been underway in Tupolev’s design bureau. The ANT-23 had a highly original concept: The crew and the two engines, one tractor and one pusher, were installed in a small nacelle. The long barrels of the recoilless APK-4 guns actually formed the tail booms. However, when a shell exploded in one of the guns the ANT-23 barely landed safely, and the aircraft was abandoned. More promising was the ANT-29. This was a conventional, highly streamlined twin-engined monoplane, with a single 102mm DRP or two APK-8 in the fuselage. The ANT-46, a basically similar design, instead had two APK-11 guns in the wings, and apparently the design goal was to use the 100mm APK-100. But the arrest of Tupolev and the disappearance of Kurchyevskii ended the development of fighters with recoilless guns.

Tank Busters

The interest in large aircraft cannon was revived during WWII by the steady increase in the armour thickness of tanks. Most air forces discovered the need for dedicated anti-tank weapons for attack aircraft. However, experience on the battlefield showed that this was not always the best solution: Tanks were well protected against anything but a direct hit, and that was difficult to achieve. Soft-skinned supply vehicles or horse-drawn artillery were much more rewarding targets, but the power of a heavy cannon was wasted on them.

In the USSR, S.V. Ilyushin managed to convince the Politburo of the need for a modern, heavily armoured ground-attack aircraft. The first prototype of the Il-2 Shturmovik flew in 1939. With 990kg of armour, the Il-2 was always a modest performer and highly vulnerable to fighter attack, but it was well-protected against small arms fire from the ground. The Il-2 was, according to Stalin, as essential for the Red Army as bread, and about 36000 were built. This made it the world’s most built aircraft, but such production figures were necessary to compensate for the heavy losses. Instead of large cannon, the Il-2 relied on high-velocity guns of small calibre. The initial armament of 20mm ShVAK cannon was insufficient, but they soon replaced by the powerful 23mm VYa. Only in 1943 a number were equipped with the 37mm NS-37. These were considered effective enough against the German tanks, because the rear and top armour was much thinner than the front armour.

P.O. Sukhoi, with his Su-6, had unsuccessfully competed with the Il-2: Although the Su-6 was a better aircraft, it was decided not to halt production of the Il-2. This did not deter him, and in 1942 he got approval for a long-range attack aircraft to complement the short-ranged Il-2. The Su-8 was a sleek, powerful, twin-engined aircraft, again with a heavily armoured cockpit. It could be armed with either four 37mm 11P-37 cannon, or two 45mm OKB-16-45cannon. The latter were fed by clips. To assist in aiming, four 7.62mm ShKAS guns were installed in the wings. The Su-8 would have had the heaviest forward-firing armament of any WWII aircraft, but it was not put in production, because the war was nearly won. The line of thinking behind the Su-8 was continued with a series of anti-armour derivatives of the excellent Tu-2 twin-engined bomber. The second prototype of the Tu-2Sh carried a 75mm cannon, and the third carried two 20mm ShVAK cannon, two 37mm NS-37 and two 45mm NS-45. The Tu-2RShR had a 57mm RShR cannon installed in the lower fuselage. All these aircraft remained prototypes.

Initially, the Germans also opted for high-velocity cannon, but they did not have a direct equivalent of the VYa. Instead, the Junkers Ju 87G anti-tank aircraft appeared with two BK 3,7 cannon in pods under the wings, with six rounds each. The BK 3,7 was a 37mm weapon, developed from the Flak 18 anti-aircraft cannon. This armament installation proved highly successful against Soviet armour, despite the vulnerability of the obsolescent Ju 87 design. A purpose-designed attack aircraft was the Henschel Hs 129. As in the Il-2, the cockpit was an armoured box. With two Gnome-Rhone 14M4/5 radials, captured French engines, the Hs 129 was decidedly underpowered. It had a MG 17 and a MG 151/20 on each side of the fuselage, but the anti-tank cannon was carried in a fairing under the belly. It could be the 30mm MK101 or MK103, but also the BK 3,7. There were even experiments with the mighty 75mm BK 7,5, in an attempt to ensure the destruction of Soviet tanks, that carried increasingly heavy armour. Such 75mm cannon, the KwK 39 and PAK 40, also appeared on the Ju 88P, but their weight, recoil, and enormous muzzle blast were too much even for this twin-engined bomber. After the Ju 88P-1, later models switched to two BK 3,7 cannon, or one BK 5, derived from the PAK 38.

The RAF and USAAF never had an armoured ground-attack aircraft similar to the Il-2. Instead, they increasingly used fighter-bombers. Compared with the Il-2, these were more vulnerable to small arms fire from the ground, especially those with liquid-cooled engines, but on the other hand they could defend themselves succesfully against enemy fighters, and in general formed a force with superior range, speed, and flexibility. Both approaches can be defended. The Germans seems to have considered the Allied air superiority on the Western front more threathening, but then the front in the East was much longer, and air support was spread thinner.

Name	Ammunition	Rate of Fire	Muzzle Velocity	Weight
MK 101	30 x 184B (355 g)	250 rpm	960 m/s	178 kg
MK 103	30 x 184B (355 g)	420 rpm	860 m/s	146 kg
BK 3,7	37 x 263B (380 g)	160 rpm	1170 m/s	295 kg
BK 5	50 x 419R (1250 g)	50 rpm	1200 m/s	540 kg
BK 7,5	75 x (3300 g)		933 m/s	1000 kg
VYa	23 x 152B (200 g)	500 rpm	905 m/s	69 kg
NS-37	37 x 195 (735 g)	250 rpm	900 m/s	150 kg
Vickers S	40 x 158SR (1130 g)	100 rpm	615 m/s	134 kg
Moulins 6pdr	57 x 441R (3170 g)	60 rpm	790 m/s	816 kg

Fighter-bombers could still be armed with large guns, and the Hurricane Mk.IID was fitted with two 40mm Vickers S cannon in pods under the wing, with two Browning .303 retained to assist in aiming. In the North African desert it proved effective, but the type was abandoned because it was vulnerable both to enemy fighters and to light AA guns, and its armament was considered to be ineffective against the newest German tanks. An alternative was the Mosquito Mk.XVIII with an Molins 57mm cannon, but this cannon too was rated unable to cope with the armour of a Tiger tank. The Mosquito Mk.XVIII, nicknamed Tsetse, was instead sent to Coastal Command for use against U-boats. The Allies switched to rockets as anti-tank weapons, and did not consider cannon again.

Near the end of the war a new breed of attack aircraft started flying. These were aircraft with the general configuration of light bombers, but a performance closer to that of fighter-bombers. A good example was the Beech A-38 Grizzly, a compact, clean aircraft powered by two mighty R-3350 radial engines. The A-38 carried the powerful T15E1 75mm cannon in the nose, with 20 rounds of ammunition. Unfortunately for the A-38, all R-3350 engines were required for the B-29 program.

Anti-ship installations

The Mosquito Mk.XVIII was not the only example of an attack aircraft for anti-ship missions, armed with a big gun. The best known example are variants of the B-25 Mitchell medium bomber. The B-25G carried a 75mm M4 gun, and the B-25H switched to the T13E1. The M4 was an army weapon, light and compact enough to be installed in the nose of a B-25. The T13E1 was a lightened version, more adapted to aircraft installations. The weapon was effective against small vessels, but because it was manually loaded only a few rounds could be fired during an attack. Near the end of the war suitable targets became scarce, and the 75mm gun was often replaced by additional .50 machineguns.

A much more ambitious project was the Piaggo P.108A. The P.108 was Italy’s modern four-engined bomber, a neat aircraft in the class of the B-17. Only a handful were built, and they equipped the only strategic bomber squadron of the Regia Aeronautica. The P.108A — A for Artigliere — carried a naval 102mm cannon in the nose, angled slightly downwards. With the recoil system, this gun weighed no less than 1500kg, and a generous 50 rounds of ammunition were provided, adding another ton to the installed weight. Apart from the need for some local strengthening and the compilation of new firing tables, the aircraft was considered a success. Plans for production of series aircraft, and conversion of P.108B bombers to P.108A configuration, were halted by the Italian surrender.

Heavy Bomber Interceptors

For the use of heavy cannon against bombers two approaches were possible. The cannon could be mounted fixed and aimed in the same way as conventional fighter armament, or it could be made flexible, and aimed either manually or by a powered system. The latter approach required a rather large and heavy aircraft, but this was less of an objection for a fighter armed with 37mm or even heavier cannon, because it was already expected to be unwieldy. The perceived advantage was the greater accuracy that could be achieved with sophisticated gunsights and automatically aimed guns. This would make it practical to open fire from a large distance, well outside the range of the defensive armament of bombers. The effectiveness of the latter was generally overestimated during the 1930s.

The most famous example of such a fighter is the Bell FM-1 Airacuda, a twin-engined aircraft first flown in 1937. The FM-1 was a low-wing monoplane with pusher propellers, driven by turbosupercharged Allison V-1710 engines. This arrangement left room for a 37mm M4 cannon in the nose of each engine nacelle, hydraulically controlled, with a coaxial machinegun to assist in aiming. A gunner in the nose of the FM-1 used a Sperry autopilot, a fire control system originally developed for anti-aircraft cannon, and an optical sight to aim these weapons. Impressive it was, but nobody could find a real need for it, or invent suitable tactics for its use; and the FM-1 faded into obscurity. There were attempts to revive the concept, for example the British Vickers 414, armed with a 40mm Vickers S cannon, but they remained paper designs.

Bell had become enthusiast about the 37mm cannon, and it proposed such armament for its new single-seat fighter, the P-39 Airacobra. This small fighter was also intended as a high-altitude bomber interceptor. As initially designed it had a turbosupercharged V-1710 engine, similar to those of the FM-1, but the gun was installed fixed, in the nose. The engine was put behind the pilot to make room for the gun, and drove a tractor propeller by a long extension shaft. Deletion of the underdeveloped turbosupercharger installation turned the P-39 into a mediocre medium-altitude fighter, and in combat the M4 revealed its weaknesses of low firing rate and modest ballistic performance. The weapon, in its improved M10 version, was also retained by the later P-63 Kingcobra, with exception of one P-63D that had the far more powerful M9. The USAAF had no use for these fighters, and both the P-39 and P-63 were mainly produced for Lend-Lease to the USSR, where they were surprisingly successful, thanks to good performance at low altitude and a high equipment standard. The M4 continued to feature in fighter designs, such as the McDonnell P-67, proposed with six such weapons in the wing roots! But the only other fighters to enter service with the M4 were the first Lockheed P-38 Lightings, and in this aircraft it soon gave way to the 20mm Hispano. This was a much more useful weapon for a fighter.

Soviet enthusiasm for the P-39 and P-63 may be related to their own use of 37mm and 45mm weapons in fighters. The Yak and LaGG fighters were powered by the Klimov M-105, a Soviet development of the French Hispano-Suiza 12Y. This engine was suitable for installation of a cannon on the centreline, firing through the propeller hub. Initially this was the 20mm ShVAK or the far more powerful 23mm VYa, but soon heavier weapons were considered. The initial choice was the 37mm Sh-37, installed in a small number of LaGG-3 and Yak-9T fighters, but soon replaced by the competing NS-37. A small number of Yak-9TK and Yak-9K aircraft received the NS-45, a straightforward modification of the NS-37 to fire a larger shell. Later the NS-37 was replaced by the lighter N-37. That these weapons also could be carried by the final Yakovlev piston-engined fighter, the Yak-9P, indicates that they were considered a success. Yet their recoil was such that the pilots were trained to fire three-round burst with the NS-37, and single shots with the NS-45. They were also instructed to do so only at high airspeeds. The pilots were carefully selected for their shooting ability, and perhaps this contributed to the relatively high effectiveness of these weapons. Experiments with the NS-57 in the Yak-9 lead to the conclusion that the recoil of this weapon was really too high, and it was not installed in production aircraft.

Heavy cannon could be carried more easily by heavier fighters, especially twin-engined fighters. Although prototypes of twin-engined fighters were built in the USSR, their production fighters were single-engined and diminutive in size. Germany, on the other hand, built a large number of twin-engined fighter aircraft, and equipped them with heavy cannon in an attempt to combat the Allied heavy bombers. This started with subtypes of the Messerchmitt Bf 110, the Bf 110G-2/R1 and Bf 110G-4a/R1, that carried the BK 3,7 cannon in a ventral gondola. The Me 410 was developed as a more powerful replacement for the Bf 110, and in line with this the Me 410A-2/U4 and Me 410B-2/U4 carried the 50mm BK 5. The appearance of long-range escort fighters made both the Bf 110 and Me 410 far too vulnerable to operate in daylight, and removed this threat. An alternative was the installation of 50mm cannon in the Me 262jet fighter, and prototypes of this aircraft with the BK 5 and the MK 214A were produced. Development of a light, low-velocity 55mm cannon, the MK 112, was never completed. The Me 262 also pioneered the use of the R4M rocket, an unguided air-to-air weapon. Rockets had been used for air combat before, mainly by the Russians, but it was the German development and successful use of the R4M that finally doomed heavy cannon as fighter weapons. They were a far more efficient, if less accurate, way to get a heavy warhead to the target.

The other nation to make widespread use of twin-engined fighters was, remarkably enough, Japan. For the Army the Kawasaki Ki.45 and Ki.102 twin-engined fighters were developed, and they were produced both in escort fighter, heavy fighter, ground attack, and anti-shipping versions. They were equipped with a surprisingly wide array of heavy cannon, including the 37mm Type 94, Ho-203, and Ho-204, the 40mm Ho-301, and the 57mm Ho-401. The 37mm Type 94 was also installed in a the fighter derivative of the Mitsubishi Ki.46 high-altitude reconnaissance aircraft, in an upward-firing installation. This was a desperate measure, for the light construction and slow climb of the Ki.46 made it unsuitable as a fighter. In addition, the Japanese adopted a 75mm anti-aircraft cannon for airborne use as the Type 88, and installed this in a fighter derivative of the Ki.67 bomber, the Ki.109. The motivation for many of these experiments was to shoot down the B-29 bomber, a large and well-defended aircraft. Most of them could be considered failures in this role, simply because they could barely reach the operating altitudes of the B-29. In any case, this was mostly a story of too little, too late.