The Westland Welkin was a British twin-engine heavy fighter from the Westland Aircraft Company, designed to fight at extremely high altitudes, in the stratosphere; the word welkin meaning "the vault of heaven" or the upper atmosphere. First conceived in 1940, the plane was built in response to the arrival of modified Junkers Ju 86P bombers flying reconnaissance missions, which suggested the Luftwaffe might attempt to re-open the bombing of England from high altitude. Construction was from 1942 to 1943. The threat never materialised; consequently, Westland produced only a small number of Welkins and few of these flew.

A heavy fighter is a historic category of fighter aircraft produced in the 1930s and 1940s, designed to carry heavier weapons or operate at longer ranges than light fighter aircraft. To achieve performance, most heavy fighters were twin-engine, and many had multi-place crews; this was in contrast to light fighters, which were typically single-engine and single-crew aircraft. In Germany, these larger fighters were known as Zerstörer ("destroyer").