Avro had already started work on a corresponding design prior to having received a formal invitation to tender. The company was in competition with Boulton Paul, Bristol, Fairey, Handley Page and Shorts. Vickers also had its Warwick, which was outfitted with Napier Sabre engines, but eventually chose against tendering it. In early 1937, both the Avro design and the rival Handley Page HP.56 were accepted and prototypes of both ordered; but in mid-1937, the Air Ministry exercised their rights to order the types "off the drawing board". This skipping of the usual process was necessary due to the initiation of a wider expansion of the RAF in expectation of large scale war in Europe. From 1939, it was expected that the P.13/36 would begin replacing the RAF's existing medium bombers, such as the Armstrong Whitworth Whitley, Handley Page Hampden and Vickers Wellington.