Design and Development
The idea of a fighter development of the Beaufort was suggested to the Air Ministry by Bristol. The suggestion coincided with the delays in the development and production of the Westland Whirlwind cannon-armed twin-engine fighter. Bristol made proposals of a fixed four cannon version and a turret fighter with twin cannons; the former was preferred by the Assistant Chief of the Air Staff. As a torpedo bomber and "general reconnaissance" aircraft the Beaufort had moderate performance but for fighter-like performance Bristol suggested their new Hercules engines in place of the Beaufort's Taurus (another Bristol engine).
Since the "Beaufort Cannon Fighter" was a conversion of an existing design, development and production could be expected far more quickly than with a completely fresh design. Accordingly, the Air Ministry produced draft Specification F.11/37 written around Bristol's suggestion for an "interim" aircraft pending proper introduction of the Whirlwind. Bristol started building a prototype by taking a part-built Beaufort out of the production line. This conversion would speed the process - Bristol had promised series production in early 1940 on the basis of an order being placed in February 1939 - and the Ministry ordered two prototypes from the line and two built from scratch. Although it had been expected that maximum re-use of Beaufort components would speed the process, the fuselage needed more work than expected and had to be completely redesigned. As such the first prototype flew for the first time on 17 July 1939, a little more than eight months after the design had started, possibly due to the use of much of the Beaufort's design and parts. A production contract for 300 machines "off the drawing board" had already been placed two weeks before the prototype F.17/39 even flew.
The first prototype achieved 335 mph (539 km/h) at 16,800 ft (5,120 m), the second prototype when laden with operational equipment was slower at 309 mph at 15,000 ft. Large orders were placed with the start of the Second World War but this meant an expected shortage of Hercules engines. In February 1940, conversion of three aircraft to Merlins was ordered; success with the design was expected to lead to production aircraft in 1941. The engine installations were the same as the Avro Lancaster outboard being matched to the Beaufighter wing through an extra section. The first Merlin powered aircraft flew in June 1940.
In general, the differences between the Beaufort and Beaufighter were minor. The wings, control surfaces, retractable landing gear and aft section of the fuselage were identical to those of the Beaufort, while the wing centre section was similar apart from certain fittings. The bomb bay was omitted, and four forward-firing 20 mm Hispano Mk III cannons were mounted in the lower fuselage area. These were initially fed from 60-round drums, requiring the radar operator to change the ammunition drums manually—an arduous and unpopular task, especially at night and while chasing a bomber. As a result, they were soon replaced by a belt-feed system. The cannons were supplemented by six .303 in (7.7 mm) Browning machineguns in the wings (four starboard, two port, the asymmetry caused by the port mounting of the landing light). This was one of the heavier, if not the heaviest, fighter armament of its time, exceeded only by the later marks of the American B-25 medium bomber with up to fourteen forward-aimed 12.7mm machine guns. When Beaufighters were developed as fighter-torpedo bombers, they used their firepower (often the machine-guns were removed anyway) to suppress flak fire and hit enemy ships, especially escort and small vessels. The areas for the rear gunner and bomb-aimer were removed, leaving only the pilot in a fighter-type cockpit. The navigator/radar operator sat to the rear under a small Perspex bubble where the Beaufort's dorsal turret had been.
The Bristol Taurus engines of the Beaufort were not powerful enough for a fighter and were replaced by the more powerful Bristol Hercules. The extra power presented problems with vibration; in the final design they were mounted on longer, more flexible struts, which extended from the front of the wings. This moved the centre of gravity (CoG) forward, an undesirable feature in aircraft design. It was moved back by shortening the nose, as no space was needed for a bomb aimer in a fighter. This put most of the fuselage behind the wing, and restored the CoG back where it should be. With the engine cowlings and propellers now further forward than the tip of the nose, the Beaufighter had a characteristically stubby appearance.
Production of the Beaufort in Australia, and the highly successful use of British-made Beaufighters by the Royal Australian Air Force, led to Beaufighters being built by the Australian Department of Aircraft Production (DAP) from 1944 onwards. The DAP's variant was an attack/torpedo bomber known as the Mark 21: design changes included Hercules VII or XVIII engines and some minor changes in armament.
By the time British production lines shut down in September 1945, 5,564 Beaufighters had been built in Britain, by Bristol and also by Fairey Aviation Company at Stockport and RAF Ringway (498); Ministry of Aircraft Production (3336) and Rootes at Speke (260).
When Australian production ceased in 1946, 365 Mk.21s had been built.
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