During 1945, in response to an approach made by the British Ministry of Supply, which had been seeking design submissions from aviation companies for an envisioned new long range civil flying boat, which was to operate in BOAC's fleet to operate its transatlantic passenger services, Saunders-Roe decided to submit a bid based upon the earlier collaborative design specification. Following evaluations of a range of different propulsion methods and configurations, the design team selected the in-development Bristol Proteus – one of the first turboprop engines – to meet the enormous power requirements of the proposed giant flying boat. In May 1946, the bid made by Saunders-Roe was selected as the winner, leading to the company receiving an order for the production of three "SR.45" flying boats.