The Avro design used the Rolls-Royce Vulture 24-cylinder X-block engine, which was two Rolls-Royce Peregrine Vee cylinder blocks mounted one on top of the other, the bottom one inverted to give the "X" shape. When developed in 1935, the Vulture engine had promise — it was rated at 1,760 hp (1,310 kW) but it proved woefully unreliable and had to be derated to 1,480–1,500 hp (1,100–1,120 kW). Avro's prototype Manchester "L7246", was assembled by their experimental department at Manchester's Ringway Airport and first flew from there on 25 July 1939, with the second aircraft following on 26 May 1940. The Vulture engine was chosen by Avro and not stipulated by the Air Ministry as is sometimes claimed; other engine layouts considered included the use of two Bristol Hercules or Bristol Centaurus radial engines. The Handley Page HP.56, always intended as the backup to the Avro, was redesigned to take four engines on the orders of the Air Ministry in 1937, when the Vulture was already showing problems.