One of three ex USAAF P-39Cs shipped to the UK for trials of its 37mm cannon. The RAF had specified the 20mm Hispano-Suiza cannon for the P-39s it had received earlier but had quickly rejected. Those aircraft were shipped to the Soviet Union and there were no Airacobras in RAF service when the trials aircraft arrived. The 37mm cannon was being assessed as a ground attack weapon. The test trio were apparently scrapped at the end of the trial. Photo from: Imperial War Museums