on the second prototype. This nearly caused a disaster when an RAF pilot dived it to the limit, causing the fabric on the top surface to tear off. Whirlwind Petter's next fixed-wing aircraft design was a radical departure from the Westland's typical high-wing fabric-covered airframe. The Westland P9 was a low winged twin-engined aircraft employing the latest technology. It was designed to meet Air Ministry specification F.37/35, which called for a single-seat cannon-armed fighter, at least 40 mph faster than a contemporary bomber and not less than 330 mph at 15,000 ft. To obtain this performance Petter and Davenport chose to minimise drag; the two Rolls-Royce Peregrine engines were fitted in closely streamlined nacelles, and their radiators were fitted inside the wing inboard sections (something that would be later copied in the de Havilland Mosquito and the Hawker Tempest I). In the two prototypes the engine exhaust was routed through the fuel tanks in the wings to reduce parasitic drag. The Air Ministry thought that this was dangerous and insisted that conventional exhaust stacks be fitted. The airframe was of thin-walled stressed skin construction, with the rear fuselage skinned in magnesium alloy. Like the Lysander, it made extensive use of extrusions in the airframe. To reduce the landing distance the wing incorporated automatic Handley Page slats coupled to the Fowler Flaps, with the radiator grilles also coupled to the flap control, which was advanced at the time. The prototype first flew in September 1938, and while it was one of the fastest and most heavily armed fighters of its era, faster than the Spitfire Mk 1, its development was problematic and protracted. The engines overheated, the hydraulic engine controls were imprecise, the slats slammed open, and production was slow. Petter was frustrated by its lack of operational status in the RAF. In November 1940, he wrote a memo to Sholto Douglas stating "The Whirlwind is probably the most radically new aeroplane which has ever gone into service... New ideas I am afraid, even with the greatest care, always mean a certain amount of teething trouble... I really do not think these troubles have been any worse than they were on, say, the Spitfire... " In reply Sholto Douglas wrote, "... it seems to me that your firm is concentrating on producing large numbers of Lysanders, which nobody wants... instead of concentrating on producing Whirlwinds which are wanted badly." Shortly after