 In August 1934, Vickers proposed to use either the Pegasus or the sleeve-valve Bristol Perseus engines instead of the evaporative-cooled Goshawk, which promised improvements in speed, climb rate, ceiling, and single-engine flight capabilities without any major increase in all-up weight; the Air Ministry accepted the proposed changes.Other refinements of the design had also been implemented and approved, such as the adoption of variable-pitch propellers, and the use of Vickers-produced gun turrets in the nose and tail positions.By December 1936, the specification had been revised to include front, rear, and midship wind-protected turret mountings.