Like many aircraft in service at the beginning if WW2, the Bristol Blenheim originated from a civilian project : an airliner. In the early 1930's, when Lord Rothermere asked the British aeronautical industry to build the fastest passenger aircraft in the world, the Bristol company was amongst those who responded to the challenge.

Bristol's response came in the shape of the Type 142 plane, which met all demands. It flew for the first time in 1934, under the name of Britain first. The aircraft was a modern, low wing, all-metal monoplane, with retractable undercarriage. It could carry up to eight passengers, had good flying characteristics and could achieve a top speed of 490 km/h at 4800 meters. When the British Air Ministry realized the Type 142 was faster than anything they had in service, they issued the B.28/35 specification, asking for a bomber version with the same qualities to be built. Such was the confidence in the Bristol company that an order was placed by the Air Ministry even before one single aircraft was built. The first production model, designated as the Bolingbroke served as the first and only prototype, since it wasn't very different from the Type 142 : the wings were repositioned to mid fuselage, the Mercury 6.S.2 640hp radial engines were replaced by superior 840hp Mercury 8s and an internal bomb bay was added under the spar.

In 1937, the aircraft had received the final name of Bristol Blenheim, and the first version ( Bristol Blenheim I ) entered production. Some changes had been made to the original design : both wings had to be mounted higher on the fuselage, in order to leave room under the spar for an internal bomb bay, and defensive armament was added as well.