[9] Advances in aircraft design achieved during the 1920s and 1930s had resulted in a generation of multi-engined monoplane bombers that were substantially faster than their contemporary single-engined biplane fighters then in service.

The separation of the tasks of flying the aircraft and firing the guns would allow the pilot to concentrate on putting the fighter into the best position for the gunner to engage the enemy.

[9] The earlier Hawker Demon biplane had tested the concept with 59 of the fighters, which had been manufactured by Boulton Paul under a sub-contract, having been equipped with a hydraulically-powered rear turret, while a number of aircraft already built were also converted as such.

[10] In April 1935, the Air Ministry released Specification F.9/35, which required a two-seater day and night "turret fighter" capable of 290 miles per hour (470 km/h) at 15,000 feet (4,600 m).

[9] In particular, the powered turret was to offer considerable flexibility, possessing both a 360-degree upper hemisphere field of fire and the ability to engage enemy bombers from a range of quarters, including below the aircraft itself.

[9] The proposed fighter was similar in size and appearance to the more conventional Hawker Hurricane, differing in weight primarily due to the use of turret-based armaments.

Furnished with a 1,030 hp (768 kW) Rolls-Royce Merlin I and initially without turret, the aircraft bore a great resemblance to the contemporary Hawker Hurricane, although it was at least 1,500 pounds (680 kg) heavier.

[17] Production orders had been prepared for the Hotspur, the initial front-running submission but Boulton Paul's turret design had gained the attention of the Air Ministry.

Hawker's progress on the project had been delayed by their commitments on other aircraft programs including the more conventional Hurricane; thus the prototype Hotspur, K8309, did not conduct its maiden flight until 14 June 1938.

[17] On 30 July 1939, the first production Defiant, L6950, conducted its maiden flight; it commenced official trials with the Aeroplane and Armament Experimental Establishment (A&AEE) in September that year.

In response to a service request which sought greater performance, the Defiant Mk II, powered by the 1,260 hp Merlin XX engine, was promptly developed.

A version of the Defiant for Fleet Air Arm (FAA), it had leading edge slats and a deeper fuselage, for the lower landing speeds required of carrier aircraft.

Tankage for up to 104 imp gal (470 L) of fuel was housed within the wing centre section along with a large ventral radiator that completed the resemblance to the Hawker fighter.

The fuselage was built in sections that were subsequently bolted together, a manufacturing method previously used on other Boulton Paul-designed aircraft which they thought made the riveting process easier.

[9] The gunner could rotate the turret directly forward and transfer firing control of the guns to the pilot, with the guns firing along each side of the cockpit canopy; this was rarely done as the turret's minimum forward elevation was 19° and the pilot did not have a gunsight, possibly because the Defiant was outfitted to perform zero deflection shooting,[26] as were several contemporaneous designs arising from Air Ministry specifications.

As flying and the guns were operated by different people, there was a need to communicate; the gunner was distracted from his firing by having to direct the pilot over the intercom.

An elevation of +19° combined with ballistic properties of .303 (7.7mm) Brownings and the Defiant's operational speed made 'line of sight' aiming – as practised by Luftwaffe pilots – a practical proposition.

Frederick "Gus" Platts, an air gunner who served in 230, 282 and 208 squadrons, stated: "The Rhino suit we had to wear on Defiants was a bear but I couldn't come up with an alternative, even though it killed dozens of us.

Initial training, formal squadron acceptance, and development of tactics began with other aircraft as it received its first Defiants only in early December at Martlesham Heath.

[34] The following day, in a patrol that was a repetition of the first, Defiants claimed four Junkers Ju 87 Stuka dive bombers, but were subsequently attacked by a flight of Bf 109Es.

By flying in an ever-descending Lufbery circle, Defiant crews sacrificed the advantage of height but eliminated the possibility of attack from underneath, while giving 360° of defensive fire.

On 19 July, seven out of nine 141 Squadron Defiants sent to cover a convoy off Folkestone were shot down by Bf 109s of JG 51, and the remaining two survived, one badly damaged, thanks only to the intervention of Hurricanes of 111 Sqn.

Actual German losses were no more than 12–15 enemy aircraft; the turret's wide angle of fire meant that several Defiants could engage the same target, leading to multiple claims.

They shot down two Do 17s but were then engaged by a Bf 109, which set their Defiant on fire; they managed to shoot down the German fighter before making a forced landing; they were awarded a bar to the Distinguished Flying Medal.

[45][N 5] The squadron lost a further five aircraft (to JG 26) on 28 August, with nine crew killed, and effectively ended operations, withdrawing to RAF Duxford the following day.

[51] However, aviation author John Taylor noted that during the Blitz on London of 1940–41, the four Defiant-equipped squadrons were responsible for shooting down more enemy aircraft than any other type in the theatre.

[53] After trials in 1940 with the School of Army Co-operation to assess its capabilities in that role, the Defiant was tested as a high-speed gunnery trainer with the Air Ministry agreeing to continue production.

[61] A high-speed target tug variant, the considerably modified turretless Defiant Mk III, was developed in response to a growing demand.

In this final target-towing variant, the Defiant ended up with a number of overseas assignments with both the RAF and Fleet Air Arm in the Middle East, Africa and India.

More than 50,000 man hours went into its production and it was unveiled in 2003 marking 60 years since the last Defiant flew out of Pendeford (the World War II training airfield next to the Boulton Paul Factory in Wolverhampton).