Development of these engines was interrupted in April 1941, when the company's factory was bombed, and on 3 October 1941 the project was cancelled by the Air Ministry.

[3] Flight testing began in 1938 using an Armstrong Whitworth Whitley II, serial number K7243, during which cooling problems were encountered with the rear row of cylinders.

Development work on the early engines was cancelled by the Air Ministry on 23 April 1941, but running of the Mk III was allowed to continue until 3 October 1941; at this point all records were ordered to be handed over to Rolls-Royce.

A projected increased capacity variant known as the Boarhound was planned but never built,[4] and a related much larger design, the Wolfhound, existed on paper only.

The latter engine featured six banks of four cylinders, a displacement of around 61 litres (3,733 cu in) and a projected takeoff power rating of 2,600–2,800 horsepower (1,900–2,100 kW).