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The Mk XII was the first production version of the Spitfire to use the Rolls-Royce Griffon engine. The Griffon was a development of the “R” sprint engine used in the Supermarine Schneider Trophy racing seaplanes of the late 1920s and early 1930s. It produced significantly more power then the equivalent Merlin engines without a similar increase in size.
The prototype that became the Mk XII was developed as a Mk IV. Under that designation it first flew in November 1941. Soon after that it was redesignated as the Mk XX, in order to avoid confusion with the photo reconnaissance PR Mk IV. Finally, in April 1942 this aircraft (DP845) received a third designation as a Mk XII.
The prototype was based on a Mk II airframe. The Merlin engine was replaced with a Griffon II with a single-stage two-speed supercharger, producing 1,735 hp. The airframe needed strengthening to cope with the increased power and torque of the new engine.
The resulting aircraft was very fast at low levels, reaching a speed of 372 mph at 5,700 feet and 397 mph at 18,000 ft. It was faster than the Mk IX up to about 20,000 feet, but above that height it was slower. The Mk XII was strictly an interim design, and for once (unlike the Mk V and Mk IX) did not enter mass production. Only 100 were built, equipping two squadrons – No.41 received its Mk XIIs in February 1943, and No.91 in April 1943. The low level performance of the Mk XII was very useful when dealing with low level hit and run raids mounted by the Fw 190, and later helped against the V-1. However, it was only with the Mk XIV that the Griffon powered Spitfires proved their potential.
