Sydney Camm and his team responded to Specification F.18/37 which called for a single-seat interceptor, with two tenders. One was referred to as the "R-type" and the other as the "N-type". These were to use fundamentally similar airframes and differ essentially in the type of engine installed. The former, which was to be named Tornado, initially appeared the most promising and was powered by a 24-cylinder X-type Rolls-Royce Vulture - basically two 12-cylinder Peregrine V engines driving a common crankshaft. The first of two prototypes was flown on 6 October 1939 with a 1760hp Vulture II, provision being made for an armament of 12 7.7mm machine guns. The similarly-powered second prototype, flown on 5 December 1940, had provision for a four 20mm cannon armament. A Tornado assembly line was laid down by Avro, the production model being intended to receive the Vulture V of 1980hp, with which both prototypes were reengined. The Vulture, meanwhile, was suffering various problems, such as connecting rod bolt failures, and as the Merlin possessed absolute priority, Rolls- Royce .was forced to abandon the production of this X-type engine. This led to cancellation of the initial production batch of 201 Tornados, only one production example being completed and flown on 29 August 1941. Approval was also given for completion of a third Tornado prototype with a 2210hp Bristol Centaurus CE.4S 18-cylinder air-cooled radial engine, this flying on 23 October 1941.