In 1939 the only aircraft available to Bomber Command were twin engined and included
the Whitworth Whitley, Bristol Blenheim, Handley Page Hampden, and Vickers Wellington.

As the war progressed the swift, twin-engined de Havilland Mosquito and four engined heavies became available,
first the Short Stirling and Handley-Page Halifax, and later the Avro Lancaster.