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 Luftwaffe's 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 It was withdrawn from daytime operations for use as a night fighter and found success in combination with the use of airborne radar to locate the enemy.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".