At the end of the first World War, the Coventry Ordnance Works were producing a 37-mm cannon for aircraft use, known as the 1½ pdr COW gun. This was a fairly large weapon, weighing 95 kg, and firing a 37 × 190 cartridge. It was relatively slow-firing at 100 to 120 rpm, and the muzzle velocity was 580 to 610 m/s. This was a modest performance, similar to that of the 37 mm M4 cannon which armed many P-39 Airacobra and P-63 Kingcobra fighters during WWII.

The design of the COW gun (and the later M4) was compromised by the necessity to keep both the weight and the recoil forces down, to allow installation in aircraft. The COW gun was a long-recoil design, with the recoil spring wrapped around the barrel, and the recoil blow was only about 2000 lb (910 kg). Nevertheless, the weapon was too big and heavy for the conventional fighters of the period. More so because, unlike the Vickers machine guns used as standard fighter armament at the time, the cannon could not be synchronized to fire through the propeller.

In October 1918, the COW gun was installed in the rear cockpit of an Airco D.H.4, fixed and aimed up. The angle of the installation in at least one (A2168) was about 80 degrees. The gun fired through a hole in the upper wing. The idea was to use this weapon against the large German bombers and Zeppelins, and after some reinforcement to prevent blast damage, testing was successfull. Three D.H.4s with COW guns were put into service. They carried a crew of two; the gunner in the aft cockpit not only loaded the weapon �?the breech was inconveniently close to the cockpit floor �?but also fired it on a signal from the pilot, who aimed with a ring-and-bead sight. Though two were sent to France, they did not see any combat before the war ended. (A2168 is recorded as flying a sortie against German Gotha bombers in August 1917, but that was presumably still without the COW gun.)

Airco DH 4

Westland took advantage of the overlap between specifications F20/27 and F29/27. Its design for F20/27 was a fairly attractive low-wing, strut-braced monoplane, with fixed landing gear. Although it did not attract orders (the F20/27 competition was won by the Gloster Gauntlet), the aircraft was an advanced design for its time. The offering for F29/27 was an obvious derivative, with the COW gun installed in the right-hand side of the cockpit. Unfortunately, the Westland COW gun fighter failed to live up to its promise: The handling qualities were apalling, with a strong and at high speed uncontrollable tendency to roll, a “disquieting�?vibration of the tail surfaces, and sluggish aileron response.

Vickers' submission for F20/27 was even more advanced than Westland's offering. The Vickers 151 Jockey was a low-wing monoplane with a cantilever wing, although it still had fixed landing gear. Technically, it was well ahead of both the Westland F20/27 and the contemporary Boeing P-26. Yet the Vickers 161 offered for F29/27 was not only entirely different, it was a throwback to a completely outdated concept. The Vickers submission was a heavily strut-braced, pusher biplane; as in the Vickers F.B.5 “Gun Bus�?of the First World War, the engine was put behind the pilot, with a propeller caged between tail booms, to leave the front fuselage available for the weapon. There were some concessions to modernity: Light alloy was used for the construction, the numerous struts were broad-chord and nicely faired, and a curious conical fairing behind the pusher engine and propeller attempted to reduced drag.

The COW Mk.III gun was installed at an angle of 45 degrees, to the right of the pilot, whose cockpit was set off to the left to keep the nose short. Fifty rounds of ammunition would be carried (thirty on the prototype), all but six in horizontal racks at the sides of the cockpit from which they would have to be loaded manually. Concerns about the effect of that the recoil would have on the handling of the aircraft were addressed by Vickers, that showed them to be well within limits. Vibration was addressed by mounting the instrument panel on rubber sponges. The spent cases were collected in a box, for if they were dumped overboard they would hit and damage the propeller; maintenance crews disliked the lack of accessibility of the box and the gun. Barr & Stroud provided a periscopic gunsight, but test pilots were disatisfied by this because they had to peer down into the cockpit to use it. A gun trigger was provided on the control column, but it was so stiff that the force required to use it affected aiming.

The completion of the Vickers 161 was slow, leading to some irritation at the Air Ministry. After it made its first flight in December 1930 or January 1931, modifications to address structural and handling problems continued, and official trials were completed only in December 1932. They revealed the Vickers 161 to be a much more satisfactory aircraft than the Westland F29/27, but still one that suffered from a disappointing performance. As a combat type, it was a non-starter, and reportedly only 24 shells were fired by its gun. Ironically, the gun in the Westland F29/27 saw much more use, if only during ground trials

Westland

Vickers 151

Vickers 161