Rolls-Royce began work on a sprint version of the Spitfire's Merlin engine, to run on a special fuel mix.
[1] On 11 November 1937 an appropriately modified Messerschmitt Bf 109 V13, D-IPKY, flown by Herman Wurster, raised the world speed record to 379 mph (610 km/h).
It was still believed that a modified Spitfire, with a Merlin delivering a forecast 2,000 horsepower (1,500 kW), stood a good chance of doing better than this, so the Air Ministry decided to go ahead and issued a contract.
Internally, the gun mountings were omitted and all military equipment, including the weapons and radio fits, was removed.
Its underwing well was extended back to the trailing edge, with the inboard section of the starboard (right) flap cut off to make room.
This was some 8 inches (200 mm) less than the prototype, in order to try and keep the tip speed down and avoid drag from the creation of transonic shocks.
[1] By May 1938 the sprint version of the Merlin II, running on a special racing fuel of leaded petrol with benzol and methanol added, was achieving 2,100 hp (1,565 kW) on the test bench, for short periods.
Politically it was necessary that any record attempt should be seen to succeed, so it would not be sanctioned unless the Spitfire showed a clear lead over the current holder.
There was no possibility of modifying the Spitfire to compete with these speeds, nevertheless work proceeded on the removal of the radiator and fairing over of its duct.
[3] A detailed analysis published in that year showed that the engine power at the low altitude required would in any case be inadequate.
Internally the manual-hydraulic undercarriage retraction was made engine-driven, along with self-sealing fuel tanks and a fireproof bulkhead.
It was subsequently used a liaison craft and "high-speed hack", at one time retained for the personal use of Air Commodore Boothman.