Defiant Mk I Specifications

Crew: Pilot and air gunner

Powerplant: One 1,030 hp Rolls-Royce Merlin III twelve-cylinder liquid-cooled engine

Span: 39 ft 4 in (11.99 m)

Length: 35 ft 4 in (10.77 m)

Max Speed: 304 mph (490 km/h) at 17,000 ft (5,181 m)

Armament: Four .303 in (7.7 mm) Browning machine guns mounted in electrically-operated turret

Photo Description: Boulton Paul Defiant Mk Is of No. 264 Squadron RAF based at Kirton-in-Lindsey, Lincolnshire, August 1940.


The Boulton Paul Defiant was a two-seat, turret-armed fighter that served in a variety of roles with the Royal Air Force during the Second World War. It was designed by John Dudley North, Chief Aircraft Designer at Boulton Paul in response to British Air Ministry Specification P.9/35 issued in June 1935, which called for a two-seat fighter with its armament concentrated in a turret to replace the obsolescent Hawker Demon biplane fighter. It was believed at the time that, by separating the tasks of flying the aircraft and firing the guns, the pilot could concentrate on putting the fighter into the best position for the gunner to deliver a more accurate and concentrated burst of fire against rapidly moving targets.

Five companies submitted proposals to meet the specification but only the Hawker Hotspur and Boulton Paul P.82 designs were selected. Prototypes of each aircraft were duly ordered in late 1935. However, with Hawker already committed to the production of the Hurricane fighter, work on the Hotspur was eventually abandoned, leaving the P.82 as the sole contender. The P.82 was officially named the Defiant and an initial production order for 87 aircraft was placed on 28 April 1937. Powered by the Rolls-Royce Merlin I engine, the Defiant was an elegant low-wing cantilever monoplane of all-metal construction with inward-retracting undercarriage. It was built around the electro-hydraulically powered Type A Mk IID dorsal turret, armed with four .303 in (7.7 mm) Browning machine guns. The turret was based on a design by the French aviation company Societe d’Applications des Machines Motrices (SAMM), which had been licensed by Boulton Paul and installed in the earlier Overstrand bomber. To help alleviate the drag of the turret, aerodynamic fairings were fitted to the fuselage of the Defiant. These were pneumatically powered and could be lowered into the fuselage so that the turret could rotate freely. The machine guns were electrically fired and were prevented from damaging the propeller or tailplane by insulated cut-off points in the turret ring. The gunner could rotate the turret directly forward and transfer firing control to the pilot, but the upward angle of the guns and the lack of a gun sight made accurate firing difficult. Two telescopic radio masts were attached to the underside of the fuselage and automatically retracted when the undercarriage was extended. The first prototype, K8310, made its maiden flight from Wolverhampton Airport on 11 August 1937, without the turret installed. The turret well was initially faired over and ballast was added to compensate for the weight of the turret and its gunner. The second prototype, K8620, followed in May 1939, fitted with the improved Merlin II engine and the full turret. Although the aircraft displayed excellent handling qualities, the extra weight and increased drag of the turret limited both speed and manoeuvrability. Additional orders were placed but delays in production meant that only three Defiants were delivered to the RAF by the outbreak of war.

The Defiant Mk I entered RAF service in December 1939 with No. 264 (Madras Presidency) Squadron at Martlesham Heath and enjoyed early success during the Dunkirk evacuation. Between 27 and 31 May 1940, the squadron claimed fifty-seven enemy aircraft destroyed, of which thirty-seven were claimed on one day alone. Luftwaffe fighter pilots initially mistook the Defiant for the single-seat Hawker Hurricane due to its similar outline and were caught unaware by the rear-firing armament. However, they soon learned to attack from below or dead ahead, where the turret provided no defense, and Defiant losses quickly mounted. In an emergency, a gunner had very little chance of escaping from his turret.

During the Battle of Britain, the Defiant proved ill-suited to daylight fighter-to-fighter combat due to its limited maneuverability and lack of forward armament. On 19 July 1940, nine Defiants of No. 141 Squadron took off from Hawkinge and ordered to patrol twenty miles south of Folkestone. En route, ten Bf 109s from III./JG 51 bounced the squadron, shooting down six Defiants in rapid succession. Hurricanes from No. 111 Squadron arrived just in time to allow the remaining Defiants to make it back to Hawkinge. Ten aircrew were killed or reported missing. On 22 August, No. 264 Squadron was posted to RAF Hornchurch but withdrew from the front line six days later after the loss of 11 aircraft, five pilots, and nine air gunners. The Defiant was never committed to daylight operations within range of enemy fighters again. It went on to play a vital role as a night fighter, target tug, and air sea rescue support aircraft.