Supermarine Spitfire Mk I

The Air Ministry ordered 310 Spitfires in 1936, before the prototype had finished its trials, and that is where the trouble started. Supermarine then employed some five hundred people and was already delivering Walrus amphibians and Stranraer flying boats; its small drawing office could not produce the production blueprints fast enough. Subcontracting was unavoidable, yet parts arrived that would not fit and drawings that were not good enough to work from. The RAF received its first two production aircraft only in July 1938, and No. 19 Squadron at Duxford was the first unit to fly them that August. Cancelling the programme in favour of licence-building the Bristol Beaufighter was seriously discussed; instead 1,000 aircraft were ordered from the new Castle Bromwich factory and a further 650 from Woolston, bringing the total to 2,160 — one of the largest orders in RAF history.

The Mk I was never a fixed design but a machine under permanent revision. Above roughly 15,000 ft moisture froze the guns, so the bays were sealed and ducted with warm air drawn from the radiator, the muzzles taped over with fabric that the first rounds tore away. The two-blade wooden propeller gave way to a De Havilland three-blade two-position metal unit and, from June 1940, to constant-speed units that transformed take-off and climb. Armoured windscreens, steel plate behind the pilot, self-sealing tank coverings, a blown canopy with better all-round vision and the Martin-Baker quick-release hood all followed.

Early aircraft reached 583 km/h with a ceiling of 9,700 m. The three-blade propeller pushed both figures up, but growing weight and the IFF aerials trimmed the top speed back again: during the Battle of Britain a constant-speed Mk I was quoted at 353 mph. The real leap came from fuel. Standardising on 100 octane, imported from 1940 through seven separate sources, allowed an emergency boost that gave the Merlin III 1,310 hp for five minutes and sharply improved low-level performance. On the armament side, a pair of 20 mm Hispano cannon produced the Mk Ib — the eight-gun aircraft becoming the Mk Ia in retrospect — but jamming was so bad that the compromise of two cannon and four machine guns was adopted instead, the arrangement that would define the Mk V.

1,567 Mk Is were built: 1,517 by Supermarine between May 1938 and March 1941, and 50 by Westland. A number of airframes also became the first photo-reconnaissance Spitfires, "Cottonised" machines stripped of guns and radio and filled with fuel and cameras instead. Once the Mk II took over in the front line, surviving Mk Is went to Operational Training Units, where they taught the pilots who would fly everything that came later.