Staff Writer (Updated: 4/25/2016):
Boulton Paul already had some experience in producing such a concept by building some 59 of Hawker's Demon aircraft in the early-to-mid 1930s. These aircraft were fighter developments of the Hawker Hart and sported an enclosed gun turret at the rearward portion of the fuselage. The turrets were designed by Frazer-Nash and proved to be a sound solution, allowing for heavy armament and better gun sights to be fitted and work in conjunction with the gunner's tendencies. Early forms of the turret were manually powered and drafty but external power sources and enclosures of frame and glass were soon developed to evolve the design to its more traditional form.
The Defiant was an interesting concept to say the least. In many ways, it harkened back to the days of World War 1, where scout airplanes were being armed for self-defense while being sent out over the Front to spy on enemy ground movements. These early aircraft sported simple defensive armaments in a rear-placed cockpit, providing the rear gunner with an adequate field-of-fire from trailing enemy fighters. In practice, this concept worked but it is still interesting to compare that line of early-century thinking with developments in the inter-war years nonetheless.
The first of two Defiant prototypes went airborne on August 11th, 1937 with an order of 400 following. However, only three were actually in RAF hands by the outset of World War 2. Within time, deliveries eventually built up No 264 Squadron and made them the first operational Defiant group on December 8th, 1939. The aircraft and her aircrews would have to wait till May 12th, 1940 to sample their first taste of combat against the German Luftwaffe. Early results proved promising as the aircraft served up no fewer than 65 enemy air kills before the end of May 1940 thanks to its excellent armament load.
The bread and butter of the Defiant system lay in its hydraulically-powered rear turret. Unsuspecting enemy pilots, thinking the Defiant was susceptible from attacks from the rear, were greeted with a hail of hot lead from the 4 x 7.7mm (.303 caliber) Browning air-cooled machine guns. The machine guns were paired two to a side and the entire turret system could scan a 360-degree area above the aircraft. Visibility was somewhat hampered by the heavy framing around the glassy enclosure but in theory, the layout was deemed acceptable. Six-hundred rounds of 7.7mm ammunition were afforded each gun, making for a staggering total of 2,400 rounds available to the gunner. Bringing the guns to bear successfully on an enemy aircraft usually yielded favorable results to Defiant crews, that is, should an enemy fighter pilot be foolish enough to contend with the aircraft from the rear.