Supermarine Spitfire Mk III

Where the Mk II simply changed engines, the Mk III set out to revise the airframe. It used the Rolls-Royce RM 2SM — later the Merlin XX — whose two-speed supercharger gave 1,390 hp; span came down to 9.3 m and length grew to 9.2 m. The main undercarriage was strengthened and raked two inches forward for ground stability, doors were added that closed the wells completely, and the tailwheel was made retractable. A redesigned windscreen with an internal laminated bulletproof panel and optically flat side quarters proved good enough to be adopted on many Mk Vs and on every Mk VI.

The first Mk III, N3297, flew on 16 March 1940. Early in 1941 a Mk V, W3237, was converted to the same standard — though without the retractable tailwheel — and took over when N3297 went to Rolls-Royce; W3237 was still flying as a trials aircraft in September 1944. On the Merlin XX the Mk III reached 640 km/h at 21,000 ft, an excellent figure for 1940. Supply killed it: the limited output of Merlin XX engines was reserved for the Hurricane II series, leaving the Mk III without a powerplant, and industrial priority moved to the Mk V.

Two airframes hardly sounds like a successful programme, but the armoured windscreen, the raked and strengthened undercarriage and the retractable tailwheel all came from it. N3297 itself had a decisive second life: turned into a powerplant development aircraft at Hucknall, it was fitted with a two-stage Merlin 60 and became, as type 348, the prototype of the Spitfire Mk IX — the mark that restored parity with the Focke-Wulf Fw 190.