A contemporary of the Supermarine Spitfire and Hawker Hurricane, it was the first single-seat, twin-engined, cannon-armed fighter of the Royal Air Force.

When it first flew in 1938, the Whirlwind was one of the fastest combat aircraft in the world and, with four 20 mm Hispano-Suiza HS.404 autocannon in its nose, the most heavily armed.

By the mid-1930s, aircraft designers around the world perceived that increased attack speeds were imposing shorter firing times on fighter pilots.

[4] The eight machine guns installed in the Hurricane fired rifle-calibre rounds, which did not deliver enough damage to quickly knock out an opponent, and were dispersed at ranges other than that at which they were harmonised.

While the most agile fighter aircraft were generally small and light, their meagre fuel capacity limited their range and tended to restrict them to defensive and interception roles.

The larger airframes and bigger fuel loads of twin-engined designs were favoured for long-range, offensive roles.

Hawker offered a variant of the Hurricane, the Supermarine Type 312 was a variant of the company's Spitfire[b] and the Supermarine Type 313 a twin-engined (Rolls-Royce Goshawk or Hispano 12Y) design with four guns in the nose and potentially a further two firing through the propeller hubs if the 12T was used, the Westland P.9 had two Rolls-Royce Kestrel K.26 engines and a twin tail.

[11] Handley Page slats were fitted to the outer wings and to the leading edge of the radiator openings; these were interconnected by duraluminium torque tubes.

[14] The engines were cooled by ducted radiators, which were set into the leading edges of the wing centre-sections to reduce drag.

[15] The airframe was built mainly of stressed-skin duralumin, with the exception of the rear-fuselage, which used a magnesium alloy stressed skin.

[16][15] With the pilot sitting high under one of the world's first full bubble canopies and the low and forward location of the wing, all round visibility was good (except for directly over the nose).

Four 20 mm cannon were mounted in the nose; the 600 round/minute fire rate made it the most heavily armed fighter aircraft of its era.

L6844 first flew on 11 October 1938, construction having been delayed chiefly due to the new features and also because of the late delivery of the engines and undercarriage.

[15] L6844 was passed to RAE Farnborough at the end of the year, while further service trials were later carried out at Martlesham Heath.

[19] This careful attention to streamlining and two 885 hp (660 kW) Peregrine engines powered it to over 360 mph (580 km/h), the same speed as the latest single-engine fighters.

[11] The aircraft had short range, under 300 mi (480 km) combat radius, which made it as marginal as an escort as the Hurricane and Spitfire.

By late 1940, the Spitfire was scheduled to mount 20 mm (0.79 in) cannon so the "cannon-armed" requirement was being met and by this time, the role of escort fighters was becoming less important as RAF Bomber Command turned to night flying.

In retrospect the lesson of the Whirlwind is clear... A radical aircraft requires either prolonged development or widespread service to exploit its concept and eliminate its weaknesses.

[citation needed] Because of the low production level, based on the number of Peregrines available, no redesign of the wing was contemplated, although Westland did test the effectiveness of leading-edge slats to reduce speeds.

[31] As the performance of the Peregrine engines fell off at altitude, perhaps a function of the propeller profile and the constant speed prop controller,[32] the Whirlwind was most often used in ground-attack missions over France, attacking German airfields, marshalling yards, and railway traffic.

The rugged frame of the Whirlwind gave pilots greater protection than contemporary aircraft during crash landings and ground accidents.

[citation needed] According to P. J. R. Moyes The basic feature of the Whirlwind was its concentration of firepower: its four closely-grouped heavy cannon in the nose had a rate of fire of 600 lb./minute – which, until the introduction of the Beaufighter, placed it ahead of any fighter in the world.

[40] On recent inspection of the salvaged wreck of P6966, it was noticed that the defective tyre fitted was not of the correct size for a Whirlwind.

[42] From then on the squadron was to have considerable success with the Whirlwind while flying against enemy Junkers Ju 88, Dornier Do 217, Messerschmitt Bf 109s and Focke-Wulf Fw 190s.

While returning, the starboard throttle jammed in the fully open position and the engine eventually lost power.

The aircraft was summed up by Francis Mason as follows: Bearing in mind the relatively small number of Whirlwinds that reached the RAF, the type remained in combat service, virtually unmodified, for a remarkably long time…The Whirlwind, once mastered, certainly shouldered extensive responsibilities and the two squadrons were called upon to attack enemy targets from one end of the Channel to the other, by day and night, moving from airfield to airfield within southern England.

By 2023 an authentic cockpit section had been built and put on display at the Kent Battle of Britain Museum in Hawkinge.

[54] Plans for a 2/3 scale replica were marketed for home building in the late 1970s and early 1980s as the Butterworth Westland Whirlwind.