The Boulton & Paul P.31 Bittern was an ambitious design for a monoplane night fighter that was let down by underpowered engines.
The Bittern was designed in response to Air Ministry Specification 27/24, for a single seat night fighter. The specification assumed that any bombers found over Britain would be unescorted, and the role of the new fighter would be to break up heavy bomber formations and thus make them more vulnerable to attack.
In a period dominated by biplanes, Boulton & Paul's chief designer, John North, decided to produce a twin engined monoplane. Two significantly different prototypes were produced. Both had some features in common. They had the same flat sided fuselage, with a flat base, curved top, and streamlined appearance when seen from the side. Both had fixed landing gear, with the main wheels under the engines. The pilot sat in an open cockpit over the nose. The wings had square ends.
The two prototypes had different engines, armament and struts.
The first prototype (J7936) was powered by two Armstrong Siddeley Lynx radial engines, which were carried in the middle of the wings, and used engine cowlings but with cylinder heads exposed in the slipstream for cooling. The wings had a narrow profile close to the fuselage and at the tips, swelling at the engines. The wings were shoulder mounted. The wheels were supported by struts that connected the wheel to the engine above, and to the base of the fuselage, with another strut connecting the engine to the base of the fuselage. It was armed with two fixed forward firing Vickers guns, carried in the sides of the forward fuselage.
The first prototype made its maiden flight in February 1927, and was found to suffer from a lack of lateral control. Eventually it was discovered that this was caused by the outer wing panels flexing in flight, neutralising the effects of the ailerons. This led to the extra wing struts of the second prototype.
The second prototype (J7947) was powered by the same engines, but this time they were mounted in nacelles carried below the wing. Wingspan increased by 5ft, and the wings were about the same thickness all along. The system of struts was similar, although an extra strut was added connecting the main wheels to the outer wing. The engines were given Townend rings instead of close cowling. The wings had Handley Page leading-edge slots. The biggest change was in the armament. The second prototype was armed with two Lewis guns, one on each side of the nose. These were carried in barbettes that could change their elevation from 0 degrees (level) up to 45 degrees above horizontal. The ring-and-bead gun sight moved to match. The idea was to make it easier to attack bombers that were above the aircraft (similar to the 'jazz music' system used in later German night fighters).
Both prototypes suffered from a serious lack of engine power, having been given underpowered engines for the period (engines of around 450hp-500hp were available). They thus performed rather badly in their trials, with a top speed of 145mph without Townend rings or 152mpg with the rings. This gave it a very limited speed advantage over the Boulton & Paul P.29 Sidestrand, let along any newer bombers, and made them slower than the next generation of biplane fighters, led by the Bristol Bulldog. The project was abandoned.
Engine: Two Armstrong Siddeley Lynx radial engines
Power: 230hp each
Span: 41ft 0in
Maximum take-off weight: 4,500lb
Max speed: 145mph (first prototype), 152mph (second prototype)
Armament: Two forward firing 0.303in Vickers machine guns (fixed on first prototype, in revolving barbettes on second.