A number of aircraft were designed as possible successors to the Spitfire and Hurricane, the least successful of which was the Westland Whirlwind, a rather unconventional twin-engine day fighter which was the first of such configuration to be fielded by the RAF. In spite of a fairly decent top speed and packing a very powerful punch with four nose-mounted cannon, the Whirlwind suffered from inadecuate engines which hampered its performance overall as well as difficult maintenance and high landing speeds which proved highly inconvenient in early-war airfields. Only two squadrons received the Whirlwind, and it was in service only up to 1941 although later it was used as a fighter-bomber (nicknamed the "Whirlybomber") in cross-Channel strikes against occupied Europe. These would serve until 1943 upon which they were retired from front-line duties.

The prototype Whirlwind first flew on 11 October 1939 and entered service in June of the following year. The Mk. I variant was the original fighter version while its conversion to a fighter-bomber was designated Mk. IA with underwing bomb racks. No other variants besides these were produced.