The Spitfire III N3297 in October 1941, with standard wing tips and a Merlin 60 series engine (note the twin wing radiators) and four-bladed propeller. As a Rolls-Royce test-bed, this was effectively a Spitfire IX prototype. Spitfire N3297 was the only Mk III built. It was used as a development aircraft and was fitted for a while with the Merlin 61. To absorb the additional power a large four-bladed propeller was fitted.