The Boulton Paul Defiant is a British interceptor aircraft that served with the Royal Air Force (RAF) during World War II. The Defiant was designed and built by Boulton Paul Aircraft as a "turret fighter" to meet the RAF requirement for day and night fighters that could concentrate their firepower on enemy bombers which were not expected to have fighter escorts due to the distance from Germany to the United Kingdom. The Defiant had all its armament in a dorsal turret offering the ability to fire in most directions. The same principle was used in the Royal Navy's Blackburn Roc which was also built by Boulton Paul.
In combat, the Defiant was found to be effective at destroying bombers, the role it was designed for, but was vulnerable to the Luftwaffes more manoeuvrable, single-seat Messerschmitt Bf 109 fighters operating from bases in Northern France. The Defiant had been designed to destroy unescorted bombers by means of beam or ventral attacks and therefore lacked forward-firing armament, which proved to be a great weakness in daylight combat with fighters. It was withdrawn from daytime operations for use as a night fighter and found success in combination with the use of airborne radar to locate the enemy. It eventually equipped thirteen squadrons in this role, compared to just two squadrons as a day-fighter, though this was mainly due to slow initial production. In mid-1942 it was replaced by better performing night-fighters, the Bristol Beaufighter and de Havilland Mosquito. The Defiant continued to find use in gunnery training, target towing, electronic countermeasures and air-sea rescue. Among RAF pilots it had the nickname "Daffy".
Development
Origins
During the 1930s, the increasing speed of military aircraft posed a particular challenge to anti-aircraft defences. Advances in aircraft design achieved during the 1920s and 1930s had resulted in a generation of multi-engined monoplane bombers that were substantially faster than their contemporary single-engined biplane fighters then in service. The RAF came to believe that its new generation of turret-armed bombers, such as the Vickers Wellington, would be capable of readily penetrating enemy airspace and of defending themselves without any accompanying fighter escort, but also recognised that the bombers of other European air forces, such as the Luftwaffe, would similarly be able to penetrate British airspace with impunity.
During 1935, the concept of a turret-armed defensive fighter to counter the bomber threat emerged during a time in which the RAF anticipated having to defend Great Britain against massed formations of unescorted enemy bombers. The RAF did not expect bombers to be escorted by fighters because fighters would not have the range to reach the UK from Germany. In theory, turret-armed fighters would approach an enemy bomber from below or from the side and coordinate their fire. The separation of the tasks of flying the aircraft and firing the guns would allow the pilot to concentrate on putting the fighter into the best position for the gunner to engage the enemy. However, manually-traversed turrets were viewed as having becoming more problematic and increasingly inadequate to effectively respond to ever-faster hostile aircraft, thus there was considerable interest in using a power-augmented turret.
The earlier Hawker Demon biplane had tested the concept with 59 of the fighters, which had been manufactured by Boulton Paul under a sub-contract, having been equipped with a hydraulically-powered rear turret, while a number of aircraft already built were also converted as such. Boulton Paul and its managing director John Dudley North had gained considerable experience with defensive turrets from producing several earlier aircraft, including the Boulton Paul Overstrand bomber, and had devised a four-gun power-operated turret, the concept and development work of which would later be a core part of the Defiant design. Boulton Paul had acquired a four-gun powered turret from the French SAMM company in 1935.