With the 21 the classification of Spitfire marks swapped from Roman to Arabic numerals. The wing of the Spitfire had changed very little during the aircraft’s development. However, with the Mk 21 the elliptical wing gave way to a new one with blunted tips and changes to the flaps and ailerons. The wing was made even stronger and carried four 20mm cannon. The fuselage was redesigned and the whole airframe was heavier than previous Spitfires, meaning the undercarriage had to be made more robust. The wheels were now fully covered by undercarriage doors in flight (this had been a feature of Mitchell’s first prototype Spitfire and the prototype Mk III but was not adopted on any production Spitfire before the 21). The usual engine was a Griffon 61 or 64 with a five bladed prop, although a few were fitted with the Griffon 85 with six blades in two sets of three rotating in different directions (known as a contra-rotating propeller) to use fully the 2,375 hp of the Griffon. The Mk 21 kept the normal bulged cockpit. When first introduced the Mk 21 had bad control characteristics, these were cured by modifications to rudder and elevators. 122 were produced.