 In spite of a traditional preference of the establishment to strictly adhere to the restrictive tare weight for the aircraft established in the tender, both Pierson and Wallis firmly believed that their design should adopt the most powerful engine available.Perhaps in response to pressure from Vickers, the Air Ministry overlooked, if not openly accepted, the removal of the tare weight restriction, as between the submission of the tender in 1933 and the flight of the first prototype in 1936, the tare weight eventually rose from to .The prescribed bomb load and range requirements were routinely revised upwards by the Air Ministry; by November 1935, figures within the Ministry were interested in the possibility of operating the aircraft at an all-up weight of , which aviation author C.F.