In response to a service request which sought greater performance, the "Defiant Mk II", powered by the 1,260 hp Merlin XX engine, was promptly developed. On 20 July 1940, "N1550", the first production Defiant Mk II performed its initial flight. The Mk II featured a pressurised fuel system, additional fuel, an enlarged rudder, a deeper radiator, a modified engine mounting and elongated cowling. Once sufficient numbers of the Merlin XX engine were available, production of the improved variant commenced; in August 1941, the first production deliveries of the Defiant Mk II took place.