The Boulton-Paul Defiant was a British fighter aircraft that was one of various designs that proved ineffective in its original role but was later adapted to be successful in other missions. Rather than using a set of forward-firing guns aimed by the pilot pointing the entire airplane, the Defiant had a second crewman operating a power-driven turret of four .303 machine guns.
Its operational concept was that it would be more flexible than a forward-firing design, in approaching unescorted bombers, which it could attack from the underside or other angles that gave more flexibility in the flight path. Unfortunately, most bombers in the Battle of Britain were escorted, and the Defiant was almost helpless against the more maneuverable German Me-109 fighters. Part of the problem was that the weight of the turret and gunner reduced maneuverability. Tactical formations were developed where Defiants would fight as a mutually supported unit, but it was never effective as a day fighter, the province of the Hawker Hurricane and Supermarine Spitfire.
Having a second crewman, however, was a distinct advantage when airborne radar became practical, with the gunner monitoring the radarscope while the pilot concentated on flying. A number of Defiants made acceptable night interceptors, although as radar became smaller, they became less useful in this role. By 1942, however, it had no combat role.
Eventually, the aircraft found several niches in training, both as a place where bomber turret gunners could practice, and as a tow aircraft for targets. Especially if the turret were removed and replaced with electronics, it was a useful light platform for electronic warfare.