Per wiki:
Mk III (Type 330)
The Mk III was the first attempt to improve the basic Spitfire design and introduced several features which were used on later marks. Powered by a Rolls-Royce RM 2SM, later known as the Merlin XX, developing 1,390 hp (1,036 kW) due to its two-speed supercharger, the wingspan was reduced to 30 ft 6 in (9.3 m) and the area reduced to 220 square feet (20.4 sq m) while the overall length was increased to 30 ft 4 in (9.2 m). The strengthened main undercarriage was raked forward two inches, increasing ground stability and had flaps to fully enclose the wheels when retracted. The tailwheel was also made fully retractable. The windscreen was redesigned, with a built-in, internal laminated glass, bulletproof panel and optically flat, laminated glass quarter panels.
The first Mk III N3297 was first flown on 16 March 1940. In addition to N3297 in early 1941 a Spitfire Mk V, W3237 was converted to a Mk III, although it didn't have the retractable tailwheel. W3237 replaced N3297 when the latter was delivered to Rolls-Royce; W3237 went on to become a test aircraft and was still being used in September 1944.
Although the new Spitfire was developed to replace the earlier marks on the production lines, a decision to allocate the limited supplies of Merlin XX to the Hurricane II series meant that the Mark III lapsed. Priority then focused on the Mark V series. The Mk III with the Merlin XX was capable of a maximum speed of 400 mph (640 km/h) at 21,000 ft (6,400 m).
N3297 became the power-plant development airframe, the wings were replaced with standard Type A and the aircraft was delivered to Rolls-Royce at Hucknall. A prototype Merlin 60 two-stage engine was subsequently installed, in effect making this aircraft (renumbered the type 348) the prototype Mk IX.
https://en.wikipedia.org/wiki/Supermari ... d_variants)
Below Supermarine Spitfire Mk I converted to MK III Prototype N3297 at Boscombe Down (EGD / EGDM), UK - England 20/07/1940
https://abpic.co.uk/pictures/view/1643454
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