Supermarine Spitfire Mk III

The Spitfire Mk III (Supermarine Type 330) was the first serious attempt to redesign Reginald Mitchell's fighter once the Mk I was in service, and it never left prototype status. Its starting point was the new Rolls-Royce RM 2SM, later known as the Merlin XX, whose two-speed supercharger promised far better altitude performance than the single-stage Merlin II and III. Around that engine the team led by Joseph Smith assembled a package of aerodynamic and structural changes meant to carry the design into a new generation: the wingspan was cut to 30 ft 6 in (9.3 m) and the wing area reduced to 220 square feet (20.4 sq m), the fuselage lengthened slightly to 30 ft 4 in (9.2 m), the strengthened main undercarriage was raked forward two inches to improve ground stability, extra panels fully enclosed the wheel recesses in flight, and the tailwheel became fully retractable at last. The windscreen was redesigned with an internal laminated bulletproof panel, replacing the external slab that had added drag, and with optically flat laminated quarter panels.

The airframe used was Mk I N3297, extensively modified, and it first flew on 16 March 1940. The shorter, blunter wing improved the rate of roll, but there was concern that at a distance it might be mistaken for the Messerschmitt Bf 109, so standard-plan wings were later fitted to the prototype; the production version was to use a wing of conventional shape able to carry either cannon or machine guns — the so-called C-type or "universal" wing, which could take four 20 mm cannon, eight .303 in machine guns, or two cannon and four machine guns. With the Merlin XX the Mk III reached 400 mph (640 km/h) at 21,000 ft (6,400 m), a substantial gain over the marks then in service. The Air Ministry placed large orders and the project appeared set for success.

Two blows derailed it. The bombing of the Supermarine factories in September 1940 destroyed many of the Mk III blueprints and a set of Mk III wings under construction, setting the work back. More decisively, the limited supply of Merlin XX engines had to be allocated to the Hurricane II series, the only way of keeping Hawker's slower but more easily built fighter battle-worthy. Rolls-Royce meanwhile offered the Merlin 45, which could be installed in a Mk I or Mk II fuselage with minimal disruption: that was the way to sustain volume output without major changes on the production lines. The Mk III was kept alive into early 1941, with thoughts of building it alongside the Mk V, and was finally cancelled in March 1941; priority passed wholly to the Mk V series.

Early in 1941 a Mk V, W3237, was also converted to Mk III configuration — without the retractable tailwheel — and replaced N3297 when the latter went to Rolls-Royce. W3237 became a test aircraft, used for deck-landing trials and other experimental work by Supermarine, and was still flying in September 1944. N3297 went to Hucknall as a power-plant development airframe: standard Type A wings were fitted and, in September 1941, a two-stage Merlin 60 originally designed for the high-altitude Wellington Mk VI bomber. It flew in that form for the first time on 27 September 1941, the same month the Focke-Wulf Fw 190 became operational and the crisis that forced the RAF to halt all but the most important daylight operations over occupied Europe took hold. Jeffrey Quill described the performance increase over the Mk VB as a "quantum leap", and a second airframe, R6700, was modified for the same engine. Renumbered as the Type 348, the aircraft became the prototype Mk IX and the direct ancestor of the Mk VII, VIII and IX series.

The Mk III therefore never reached a squadron, yet almost nothing of it was wasted: the universal wing, the strengthened raked undercarriage, the wheel-bay doors, the retractable tailwheel and the new internally armoured windscreen all reappeared on the Mk Vc and later marks, and its airframe was the bridge to the two-stage Merlin Spitfires.