Produced at the Castle Bromwich “shadow” factory rather than at the Supermarine works at Southampton, production of the mark II had just got into full swing when the Supermarine works were badly damaged by bombing. The mark II was powered by the more powerful Merlin XII giving 1,175 hp increasing speed by some 10 mph. This may seem only a modest gain – but the Merlin XII used a Glycol/water mixture under pressure for cooling rather than the unpressurised pure Glycol system used on earlier Merlins. The new system removed heat from the engine more efficiently and made possible the later jumps in engine power of later Merlins. Armament was the same as the Mk I with different IIA and IIB versions with machine guns and cannons respectively. Production was 750 Mk IIA and 170 Mk IIB.