Though based on the innovative turret fighter concept first explored on the Hawker Demon and a number of contemporary French designs, the Boulton Paul Defiant ultimately proved a failure. It was designed as a two-man fighter in which the gunner would operate a 4-gun power-operated turret which would give the aircraft a greater radius of fire. Unfortunately, the gun could not fire directly forward and lacked any other type of forward-firing guns something that was soon enough discovered by the Germans who quickly learned to exploit such a weakness. Though partially successful at Dunkirk, it suffered disastrous losses during the Battle of Britain and was soon withdrawn to other roles including radar-equipped night fighter where it was relatively effective. It spent its last years of service in a number of secondary duties most notably electronic warfare were it was equipped with the Mandrel jamming system.

The prototype P.82 was first flown on 11 August 1937 and entered service in December 1939 as the Mk. I of which the F.I was a day-fighter and the NF.I a night fighter. NF.IA aircraft had an AI.Mk. IV or VI radar installed. The Mk. II had an uprated Merlin XX engine and was soon converted into a night fighter. Target tug conversions included the TT.I (based on the Mk. II) and TT.III (based on the Mk. I).