Originally designed as a bomber destroyer, the Defiant was pressed into service as a day fighter over the beaches of Dunkirk and during the Battle of Britain. Despite early successes ever
increasing losses saw the Boulton Paul Defiant moved to night fighting, where it excelled.
In 1935 a new tactical concept was conceived, this proposed the idea that fighters would have a gun turret mounted on them, as opposed to forward firing guns.
This appeared to have two advantages, firstly, it meant the pilot could concentrate on flying the aircraft, and not have to worry about finding and hitting a target.
Secondly the guns could be used over a greater area than before and could be used for both offence and defence. The idea was not completely new as it had been tried a year earlier,
but for completely different reasons on a Hawker Demon biplane, of which a number of these were built under sub-contract by Boulton Paul Aircraft.
When the Air Ministry issued specification F.9/35 for a two-seater fighter plane with a power operated gun turret, Hawker and Boulton Paul both made submissions.
Hawker's prototype was called the Hotspur, however this aircraft was not to compete against the two which were ordered from Boulton Paul. This was mainly because there was no
producing capacity left in the Hawker factories, meaning the prototype was abandoned.
The first prototype of Boulton Paul's, now called the Defiant, made it's initial flight on the 11th August 1937. A two seater low-wing cantilever monoplane fighter of all metal
construction, with retractable tail wheel type landing gear and powered by a Rolls-Royce Merlin I inline engine (the second prototype had a Merlin II engine). Both of the prototypes
had the large heavy four gun turret mounted within the fuselage, just behind the cockpit. However the weight and high degree of drag caused by the protruding section of the turret
was to limit both the manoeuvrability and speed of the Defiant.
The first production Boulton Paul Defiant was flown on the 30th July 1939 and was originally a day fighter, and in December 1939 deliveries of the plane were made to No. 264 Squadron.
It was with 264 Squadron that the Defiant's were first deployed operationally. This was on the 12th May 1940 over Dunkirk, where it achieved complete surprise, as fighters making a
conventional attack on the tail were met with a burst of fire from it's four machine guns. By the end of May 1940 they had claimed a total of 65 enemy aircraft destroyed. This air
superiority was only brief for it did not take long for the Luftwaffe pilots to work out that if they attacked the Defiant head on, they could do so with complete immunity.
This saw the aircraft suffer huge losses and as a result they were removed from daylight operations in August 1940.
The Boulton Paul Defiant was instead moved into a new night fighter role, and a number of aircraft were installed with a comparatively new and secret Airbourne Interception (AI) radar.
It was in the night fighter role that the Defiant was to have success as part of Britain's night defences in the winter of 1940-41, as during this period they were to shoot down more
enemy aircraft than any other contemporary night fighter.
To try to improve the performance of the Defiant, two MK I's served as prototypes for the new MKII version. Fitted with a more powerful Merlin XX engine, the rudder was improved with
a greater area, there were also modifications to the fuel systems and engine cooling, and also an increased fuel capacity. This new Defiant was first flown in June 1940 and over 200
were built, although many were converted to target tugs later on, and a number were built as dedicated target tugs. Also serving in other roles, Defiant's were used for air sea rescue,
carrying air dropped dinghies, and some also carried the Mandrel noise jammer to co combat the German early warning radar.
During it's peak as a night fighter, 13 RAF squadrons were using Defiant's. When converted as target tugs they were used on the home front and in the middle and far east.
The Defiant was removed from combat duties in 1942 and when production ended a year later 1,064 had been built.