SUPERMARINE SPITFIRE (Merlin, Fighter) - Single-seat high-performance fighter, initially evolved by R J Mitchell as private venture to improve on unsuccessful Supermarine 224 prototype built to Specification F.7/30 -which gave effect to the first designated Air Ministry Operational Requirement (OR. 1). By 1934, PV Supermarine 300 design had been developed to meet OR. 17 and single prototype ordered on December 1, 1934, to conform with Specification F.37/34. Powered by a 990 hp Rolls-Royce Merlin C with DH two-bladed fixed-pitch wooden propeller, unarmed prototype K5054 first flown at Eastleigh on March 5, 1936. Successively fitted with 1,035 hp Merlin F and 1,030 hp Merlin II and with full armament of eight 0.303-in (7.7-mm) machine guns, before being written off on second day of World War II, September 4, 1939.
Supermarine Spitfire I: Initial production version, built to Specification 16/36; first contract placed June 3, 1936. First aircraft flown May 14, 1938, with 1,030 hp Merlin II; Merlin III introduced at 175th aircraft. Early aircraft with two-bladed Airscrew Co wooden propeller; three-bladed metal propeller soon standardised - DH two-position, then DH or (few only) Rotol constant speed unit. Entered service August 1938 with No 19 Sqn at Duxford and nine squadrons operational by September 1939, with ten more by mid-1940. Designation Supermarine Spitfire IA adopted retrospectively in 1940 after 30 aircraft designated Supermarine Spitfire IB when fitted with two Hispano 20-mm cannon each in place of four of the machine guns. Total Mk I production (excluding conversions to prototypes of later marks), 1,519 by Supermarine and 50 by Westland. One Mk I exported to France, mid-1939; 18 ex-RAF Mk Is to Portugal in 1942. One RAAF, one RNZAF and two RCAF squadrons flew Supermarine Spitfire Is that remained on RAF strength.
Supermarine Spitfire II: Similar to Mk I but powered by 1,175 hp Merlin XII and with operational refinements progressively introduced on Mk Is. Three (or more) Mk Is flown with Merlin XIIs before first production Mk II (Supermarine Type 329) completed in June 1940 at Castle Bromwich Aircraft Factory. Operational use began August 1940 with No 611 Sqn and production totalled 751 Supermarine Spitfire IIA with eight machine guns and 170 Supermarine Spitfire IIB with two cannon and four machine guns. For extended range, Supermarine Spitfire IIA (LR) carried a 30-Imp gal (136-1) fuel tank flush-fitting under the port wing leading-edge; limited use by five RAF squadrons in mid-1941. The Supermarine Spitfire IIC (later, ASR Mk II) was adapted for air-sea rescue duties, with 1,240 hp Merlin XX in place of Merlin XII, rescue packs in the flare chutes and marker bombs under the port wing. About 50 conversions flown by RAF squadrons from 1943. Supermarine Spitfire Us from RAF stocks made available for squadrons of RAAF, RCAF and RNZAF in Fighter Command, together with, in 1941, three US-manned Eagle squadrons and units of the Forces Aeriennes Francaises Libres. Later marks flown by most of these units as they became available.
Supermarine Spitfire III: Single Mk I airframe (N3297) flown March 1940 with Merlin R.M.3SM (ie, Merlin XX) and operational/ performance improvements including structural strengthening, retractable tailwheel and short-span wing of 30 ft 6 in (9.30 m). Production plans cancelled and prototype (Supermarine Type 330) later fitted with Merlin 60/61-series two-speed two-stage engine. Second Mk III conversion (Type 348) in 1941, based on Mk V airframe and used to develop new u/c with main wheels raked forwards 2 in (1.5 cm) and, later, four-cannon wing armament.