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The Mk III was the first major redesign of the Spitfire. The new aircraft was based around the Merlin XX engine, a 1240 hp engine with a two-speed supercharger, which would have given much better high altitude performance. Other changes included a shorter clipped wing, which reduced the wing span to 32’7”, increasing the rate of roll. The bullet proof glass on the canopy was moved inside the cockpit, reducing drag. The tailwheel was made retractable. The universal “c” wing was used, which could take four 20mm cannon, eight .303in machine guns, or two cannon and four machine guns. Maximum speed increased to 385 mph.
However, although an order was placed for mass production of the Mk III, it was soon cancelled. The Merlin XX was in short supply, and was needed more urgently in the Hurricane II. Meanwhile, Rolls-Royce had developed the Merlin 45, a similar engine that could be used in a Mk I or Mk II fuselage. The Mk III was abandoned in favour of the Mk V, although many of the improvements first seen in the Mk III were used in later versions of the aircraft. The Mk III airframe was later used to test the Merlin 61 engine, becoming the ancestor of the Mk VII, VIII and IX Spitfires.
Prototypes - Mk I - Mk II - Mk III - Mk V - Mk VI - Mk VII - Mk VIII - Mk IX - Mk XII - Mk XIV - Mk XVI - Mk XVIII - Mk 21 to 24 - Photo Reconnaissance Spitfires - Spitfire Wings - Timeline