 The layout was originally tested on a converted Mk IIB and flew on 18 September 1941.A new-build version of what was known as the Mk IID started in 1942, including additional armour for the pilot, radiator and engine.The aircraft were initially supplied with a Rolls-Royce gun and carried 12 rounds, but soon changed to the 40 mm (1.57 in) Vickers S gun with 15 rounds.