The Boulton Paul P.89 was a design for a twin engined cannon armed fighter, designed in response to Air Ministry Specification F.37/35.
The original version of this specification called for a single engined fighter armed with four cannon, a top speed 40mph faster than any contemporary bombers, a retractable undercarriage, wheel brakes and an electric starter. Worries about the lack of powerful enough engines meant that the specification was altered in April 1936 to allow twin engined designs to be submitted.
Boulton Paul submitted the single engined P.88, which was briefly given a prototype contract before being cancelled, and the twin-engined P.89, which would have been powered by two Rolls Royce Kestrel XVI engines. The P.89 was up against designs from Supermarine and Westland, and it was considered to be the worst of the three. Supermarine's design was considered to be the best, but the company was too busy with the Spitfire, and so the Westland design was ordered into production as the Westland Whirlwind.