Built as a company-funded project to fill F.11/37, the prototype Beaufighter first flew on July 17, 1939. This was little more than eight months after the design had started, and hints to its widespread use of the Beaufort's design and parts. A production contract for 300 machines had already been placed two weeks before the prototype flew, as F.17/39.
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 center section was similar apart from certain fittings. The bomb-bay was faired over and used to mount a forward-firing armament of four 20mm cannons, and the areas for the rear gunner and bomb-aimer were removed, leaving only the pilot in a smoother fighter-type cockpit, and the navigator far to the rear in a small bubble where the dorsal turret used to be.
The earlier Taurus engines were replaced by the much-improved Hercules, whose extra power presented problems with vibration. In the end they were mounted on longer, more flexible struts, which stuck out from the front of the wings. This had the side effect of moving the center of gravity forward, generally a bad thing for an aircraft design. It was then moved back into place by cutting back the nose area, which was no longer needed for the bomb-aimer in the fighter role. This put most of the fuselage behind the wing and moved the CoG back to where it should be, leading to the Beaufighter's famous stubby appearance.
By fighter standards the plane was rather heavy, and rather slow. It had an all-up weight of 16,000 lbs and a maximum speed of only 335 mph at 16,800 feet. Nevertheless this was all they had at the time, as the otherwise excellent Westland Whirlwind had already been cancelled due to production problems with its engines.
The Beaufighter's main claim to fame would be that it was coming off the production lines at almost exactly the same time as the first British airborne radar sets were. With the weapons mounted in the bomb-bay, the nose area was left clear for mounting the radar antennas, and the planes were adapted as night fighters as quickly as possible. Even loaded down to an even heavier 20,000 lbs, their slow performance was more than enough to catch the even slower German bombers. By early 1941 they had put an end to Luftwaffe bad-weather and night raids.
Improved versions of the Hercules continued to improve the load capacity of the fighter, although performance didn't tend to increase. As the faster De Havilland Mosquito took over in the night fighter role, the heavier Beaufighters found use in anti-shipping and ground attack roles.
However well the Beaufighter performed, the Shorts Stirling bomber program had a higher priority for the excellent Hercules engine by late 1941, and the Rolls Royce Merlin XX powered Mk.II was the result. There were no Mk.III's or IV's, and only two Mk.V's. The Hercules returned with the next major version in 1942, the Mk.VI, which was eventually built to over 1,000 examples. The last major version (2,231 built) was the Mk.X, probably the finest torpedo and strike aircraft of its day. By the time the line shut down in September 1945, 5,562 Beaufighters had been produced, the majority of them the later models.
The Beaufighter was also operated by a variety of air forces of the British Commonwealth, including the Royal Australian Air Force, Royal Canadian Air Force, and Royal South African Air Force. Following the war, it was used by the Portuguese Air Force and briefly by the Israeli Air Force.