Other candidates were submitted for the same specification, including the Avro 679, and designs from Fairey, Boulton Paul and Shorts; all submissions were designed around two-engine configurations, using the Rolls-Royce Vulture, Napier Sabre, Fairey P.24 or Bristol Hercules engines. The majority of these engines were under development at this point; while four-engined bomber designs were considered for specification B.12/36 for a heavy bomber, wings mounting two pairs of engines were still in the experimental stage and required additional testing at the Royal Aircraft Establishment (RAE). Adopting a stronger wing also required additional strengthening of the overall aircraft structure, resulting in an increase in overall weight.