Spitfire Mk III N3297
The Spitfire III represented the first major overhaul and enhancement of the original Spitfire design. Supermarine proposed this upgraded version in February 1939, which incorporated several significant changes.
Central to the Spitfire III was the more powerful Merlin XX engine, which featured a two-speed supercharger. Its installation necessitated reinforcing the fuselage and altering the engine mounting and cowling. The aircraft’s wings were shortened, a retractable tailwheel was added, fuel capacity was increased, and armor protection was improved. Additionally, the oil cooler intake and coolant radiator under the wings were enlarged, the main undercarriage was slightly angled forward, and fairings were added to cover the main wheels when retracted. The Mk.III also introduced the universal ‘C’ wing, designed to accommodate various armament options, and it was projected to be the first Spitfire model capable of achieving 400 mph (644 km/h) in level flight.
Initially, there was a supply issue with the Merlin XX engine, as it was earmarked for the Hurricane II. However, once this supply was secured, an order for 1,000 Spitfire IIIs was placed with the Castle Bromwich Aircraft Factory in October 1940. The prototype (N3297), which had been converted from a Mk.I on the production line, made its first flight on March 15, 1940. However, shortly after placing the production order, it was decided to prioritize the Merlin 45-powered Spitfire V due to the need for minimal production disruptions. As a result, the 1,000 units on order were completed as Mk.Vs. The prototype N3297 was repurposed for testing and played a crucial role in the Spitfire development program, emerging in September 1941 with a Merlin 61 engine, a four-bladed propeller, and standard span wings, effectively serving as the prototype for the Mk.IX.
A second Spitfire III (W3237), converted from a Mk.V airframe, began flight tests in June 1941. This aircraft was used for experimenting with extended wingtips, assessing roll performance, and supporting Seafire development, including takeoff performance trials with various flap settings.