The Boulton & Paul P.35 was a design for a fighter with an upwards firing Coventry Ordnance Works 37mm cannon.
Air Ministry Specification F.29/27 called for a bomber destroyer, to be armed with the 37mm COW gun, and which was expected to attack enemy bombers from below. The 37mm COW gun was a rather clumsy weapon to be installed in a fighter aircraft, with a length of 8ft, a recoil of 2,000lb and 1.5lb shells. It took clips that carried five shells, and needed to be manually reloaded in the air.
Boulton & Paul produced two designs in response to F.29/27. The first, for a two-man biplane, didn't get a P number. It had the pilot located in front of the wings, the COW gun behind the wings, firing up and over the wings, and the gunner behind the gun.
The second design, which was submitted as the P.35, was for a low wing monoplane, powered by a Rolls-Royce F.11S (later the FXI, and the Kestrel) supercharged engine. The wings were equal chord, square tipped and supported by struts and wire bracing. Radiators for the engine were recessed into the inner wings. The P.35 had a very wide tracked non-retractable undercarriage, to make night landings safer. It also had a very long fuselage to balance the heavy COW gun.
This time the pilot also had to reload and operate the gun, which was mounted just to his right. The P.35 wasn't accepted by the Air Ministry, but prototypes were ordered from Vickers (Type 161) and Westland (C.O.W. Gun Fighter, also a low wing monoplane).