Mk II was designed as a contingency measure against an anticipated shortfall of Rolls-Royce Merlins for the Mk I, equipped instead with Bristol Hercules radial engines. The performance was not as good as the Mk I, and production was limited to just 300 after the availability of US-made Merlins was ensured. Another distinguishing feature of the early Mk Is was a row of windows on the upper fuselage, which were removed in later versions.