Supermarine Spitfire Type 300 prototype K5054

The Spitfire grew out of racing. Mitchell had spent years refining Supermarine's Schneider Trophy floatplanes, and the S.6B won the trophy outright for Britain in 1931. His first answer to the Air Ministry's fighter specification, the fixed-undercarriage Type 224, was a disappointment whose cooling system never behaved, and the ministry ordered the Gloster Gladiator biplane instead. Mitchell's team kept refining the idea on its own account, adding a retractable undercarriage, an enclosed cockpit, the new Rolls-Royce PV-12 engine that would become the Merlin, and the elliptical planform whose real virtue was not its looks but the extremely thin section it allowed.

Work on K5054 began in December 1934 and changes kept arriving as the airframe took shape, the most significant being a switch to ethylene glycol cooling in a ducted radiator derived from Frederick Meredith's research at the Royal Aircraft Establishment. The aeroplane took off for the first time on 5 March 1936 with Vickers chief test pilot Joseph "Mutt" Summers at the controls. It flew unpainted and unarmed, its undercarriage locked down and unfaired, on a prototype Merlin C rated at 990 hp. Summers' remark on landing, "don't touch anything", is usually quoted as proof the aircraft was already perfect; he simply wanted nothing altered before he had dictated his notes.

Ten days later the machine reappeared in a polished pale blue-grey finish, with faired undercarriage legs, revised cowlings, a straight-topped fin and a better propeller. Performance was now convincing enough for the aircraft to be handed to the RAF for service trials. The intended armament of eight .303 in Browning machine guns was fitted later in 1936, and in 1938 ejector exhausts were tried, recovering some seventy pounds of extra thrust from the exhaust gas. An order for 310 aircraft had already been placed on 3 June 1936.

K5054 was destroyed on 4 September 1939 at Farnborough, tipping onto its back after a misjudged landing and fatally injuring its pilot, Flt Lt White. Mitchell had been dead for two years by then, having succumbed to cancer on 11 June 1937 at the age of forty-two, and his former chief draughtsman Joseph Smith directed every later mark. The single prototype left behind an airframe healthy enough to absorb engines of twice the power and roles nobody had considered in 1936.