The legend of the Supermarine Spitfire was forged in the skies over Britain during the summer of 1940. The graceful looking aircraft made it easy for the British media spindoctors to turn R.J. Mitchell’s design in to an unbeatable weapon of war as far as the public image was concerned but this hid the truth that already its shortcomings were becoming obvious even before the Battle of Britain and the early Spitfire Mk.I and the slightly improved Mk.II would need replacing by 1941.
The Supermarine engineers returned to the drawing board and looked at almost every aspect of the aircraft taking in to account the lessons learned from the early experiences in service. Assigned the in-house designation of Supermarine Type 330 the Spitfire Mk.III would need to be faster than its predecessors in order to allow it to keep up with the latest German fighters and to achieve that the Rolls-Royce RM 2SM engine (later redesignated Merlin XX) was chosen to power the new type. This provided an additional 215hp over the Merlin XII as fitted to the Spitfire Mk.II and was the first Merlin engine to feature a two-speed supercharger.
Not satisfied with simply putting a more powerful engine in the aircraft the designers at Supermarine incorporated a number of aerodynamic improvements including the fitting of doors over the wheels that closed when they were retracted in to the wing thus preventing the space around the wheel from producing drag. Additionally the tail wheel was made to retract in to the rear fuselage and the armoured windscreen was moved inside the cockpit both of which improved aerodynamic efficiency.
Perhaps the most noticeable change was the cropping of the Spitfire’s signature pointed wingtips making them come to a flat straight line. This reduced the Spitfire III’s wing span from 36ft 10in in the Mk.I/II to 30ft 6in which improved roll rate at lower levels, something for which the Spitfire was found to be lacking compared to its rivals, but high altitude performance dropped off slightly. The reduced forward cross section of the aircraft with cropped wings also helped improve acceleration by reducing drag. The wing was essentially a modified “c” wing which allowed the aircraft to be fitted with different weapon options (see below). A less obvious change to the aircraft was a 7in fuselage extension to fit the engine and balance it out.
All these efforts resulted in the first Spitfire III being able to attain a maximum speed of 400mph at 21,000ft, almost 50mph faster than the previous Spitfires. This offered a big advantage in the air compared to some of the aircraft’s rivals in late 1940;
- Messerschmitt Bf109E – 336mph at 19,685ft
- Macchi C.202 – 372mph at 18,370ft
The first Spitfire Mk.III was N3297 and the aircraft first flew on March 16th 1940. Testing of the aircraft continued despite the constraints of the Battle of Britain that was raging overhead and the RAF was sufficiently interested in the aircraft to eventually put in place a production order for over 1,000 aircraft with deliveries to begin in early 1941. However the Merlin XX was already earmarked for use in the Hawker Hurricane Mk.II project which was seen as a higher priority largely thanks to the fact that the Hurricane needed the uprated engine more than the Spitfire to make up for its shortfalls in combat with the Bf109. The Spitfire Mk.III was not abandoned but merely put on hold and N3297 continued testing in order to perfect the design.
As 1941 came it was becoming increasingly clear that the window for the aircraft to enjoy its advantages to the fullest was rapidly closing thanks to the Merlin XXs going to the Hurricane. Also Rolls-Royce was already developing their Merlin 45 series of engines which would ultimately power the Spitifre Mk.V which was ordered in to production in late 1940 with the first examples arriving just a few months later. Despite this the Spitfire Mk.III prototype was providing a lot of useful data to Supermarine allowing them to fine-tune the airframe changes and incorporate them in to production Spitfires. A second aircraft, Spitfire Mk.V W3237, was configured to an almost identical standard (minus the retractable tailwheel) to continue Mk.III testing while N3297 was fitted with a standard “A” wing and delivered to Rolls-Royce for engine testing eventually being fitted with the first Merlin 61 engine that would be used in the Spitfire Mk.VIII and IX. Even with a second prototype the Spitfire Mk.III project was now cancelled altogether but W3237 continued testing until 1944 to support development work on newer model Spitfires.
While the Spitfire Mk.III was a non-starter the fruits of the testing carried out by the two prototypes can be seen in many of the improvements made to the later Spitfire marks such as cropped wings and doors over the landing gear. The Merlin XX did find its way in to around 50 Spitfire II aircraft converted for use in the air-sea rescue role.
ENGINE: 1 x 1,390hp Rolls-Royce Merlin XX inline engine
MAX SPEED: 400mph at 21,000ft
WINGSPAN: 30ft 6in
LENGTH: 30ft 4in
HEIGHT: 9ft 10in
WING AREA: 220ft2
- 8x .303 (7.7mm) Browning machine guns
- 2x 20mm Hispano cannons / 4x .303 (7.7) browning machine guns
- 4x 20mm Hispano cannons