The Boulton Paul Defiant is a British interceptor aircraft that served with the Royal Air Force (RAF) during World War II. The Defiant was designed and built by Boulton Paul Aircraft as a "turret fighter" to meet the RAF requirement for day and night fighters that could concentrate their firepower on enemy bombers which were not expected to have fighter escorts due to the distance from Germany to the United Kingdom. The Defiant had all its armament in a dorsal turret offering the ability to fire in most directions. The same principle was used in the Royal Navy's Blackburn Roc which was also built by Boulton Paul.
In combat, the Defiant was found to be effective at destroying unescorted bombers, the role it was designed for,Verkaik, 2020 p334 but was vulnerable to the Luftwaffes more manoeuvrable, single-seat Messerschmitt Bf 109 fighters operating from bases in Northern France, allowing them to escort bombers to London, although with fuel for only ten minutes of flying time there. The Defiant had been designed only to destroy unescorted bombers by means of beam or ventral attacks, and had no forward-firing armament; it proved to be very vulnerable to frontal attacks by fighters in daylight combat. It was withdrawn from daytime operations for use as a night fighter, and found success in combination with the use of aircraft interception radar (A.I.) to locate the enemy.Wheeler 1992, p. 48. It eventually equipped thirteen squadrons in this role,Cagill 2005, p. 44. compared to just two squadrons as a day-fighter,Cagill 2005, p. 41. though this was mainly due to slow initial production.Verkaik, 2020 p309 In mid-1942 it was replaced by better-performing night fighters, the Bristol Beaufighter and de Havilland Mosquito. The Defiant continued to find use in gunnery training, target towing, electronic countermeasures and air-sea rescue. Among RAF pilots it had the nickname "Daffy".
Development
Origins During the 1930s, the increasing speed of military aircraft posed a particular challenge to anti-aircraft defences.Bowyer 1966, p. 3. 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 RAF came to believe that its new generation of turret-armed bombers, such as the Vickers Wellington, would be capable of readily penetrating enemy airspace and of defending themselves without any accompanying fighter escort, but also recognised that the bombers of other European air forces, such as the Luftwaffe, would similarly be able to penetrate British airspace with impunity.
During 1935, the concept of a turret-armed defensive fighter to counter the bomber threat emerged during a time in which the RAF anticipated having to defend Great Britain against massed formations of unescorted enemy bombers.Buttler 2004, p. 51.Mondey 2002, pp. 40–41. The RAF did not expect bombers to be escorted by fighters because fighters would not have the range to reach the UK from Germany.Verkaik, 2020 p61 In theory, turret-armed fighters would approach an enemy bomber from below or from the side and coordinate their fire. 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. However, manually-traversed turrets were viewed as increasingly inadequate to effectively respond to ever-faster hostile aircraft, thus there was considerable interest in using a power-augmented turret.
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.Mondey 2002, p. 41. Boulton Paul and its managing director John Dudley North had gained considerable experience with defensive turrets from producing several earlier aircraft, including the Boulton Paul Overstrand bomber, and had devised a four-gun power-operated turret, the concept and development work of which would later be a core part of the Defiant design. Boulton Paul had acquired a four-gun powered turret from the French SAMM company in 1935.
In April 1935, the Air Ministry released Specification F.9/35, which required a two-seater day and night "turret fighter" capable of at . The aircraft was to feature a clean design, concentrating its armament within a power-operated turret, and the accepted performance was to be only slightly beneath that of other emergent fighter designs of the period, along with a sufficient fuel capacity to allow it to perform standing patrols. 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. Specification F.9/35 had followed the earlier Specification F.5/33, which had sought a pusher design combined with a forward-set turret; F.5/33 had been abandoned as the proposals had offered little in terms of performance gains over existing fighters, and the corresponding Armstrong Whitworth AW.34 design which had been ordered was not completed.
F.5/35 was issued to British manufacturers in June 1935. Seven responded to the tender: Armstrong Whitworth (twin-engines, based on the AW.34), Boulton and Paul (P.82), Bristol (Type 147), Fairey, Gloster, Hawker, and Supermarine (Type 305). Vickers did start on a design but did not provide it to the Air MinistryButtler, p52-53 In September 1935 the Air Ministry picked Hawker, Boulton Paul and Armstrong Whitworth designs as the top three (in decreasing order); Gloster and Bristol were rejected due to their turret designs. The Air Ministry wanted several designs investigated and the production of two prototypes of each. To get this large number of aircraft, HM Treasury had to approve the expenditure above that already allocated. Seven prototypes were ordered - one from Armstrong Whitworth, two each from Fairey, Hawker and Boulton Paul - but only the two Defiant prototypes and one Hawker were completed.Buttler, p54
P.82 Boulton Paul, having been focused on turret-equipped aircraft for some time, made a submission to Specification F.9/35 with the company designation of P.82. 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. The central feature of the P.82 was its four-gun turret, based on a design by French aviation company (SAMM), which had been licensed by Boulton Paul for use in the earlier Boulton Paul Sidestrand bomber, eventually installed in the "follow-up" design, the Boulton Paul Overstrand and in the Blackburn Roc naval fighter.Bowyer 1970, p. 270. The 'Type A' turret was an electro-hydraulically powered "drop-in" unit, with crank-operated mechanical backup. Small bombs could be housed in recesses in the outer wing. Some of the development work from the company's earlier B.1/35 tender was carried over into the P.82.
In 1936, Boulton Paul commenced assembly on the first P.82 prototype, K8310, at their new Wolverhampton facility; an order for a second prototype, K8620, was received by the following year. In 1937, the first P.82 prototype, K8310, was rolled out. 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 heavier. On 11 August 1937, K8310, which had recently received the name Defiant, conducted its maiden flight. This initial flight, still without the turret, piloted by Boulton Paul's chief test pilot Cecil Feather, took place nearly a year ahead of the rival Hotspur. Official acceptance trials did not commence until nine months later. On 30 July 1939, the second prototype, K8620, equipped with a Merlin II engine and a full turret, conducted its first flight. K8620 had received various modifications over the first prototype, such as telescopic radio masts and revisions to the canopy and to the undercarriage fairing plates; implementing these improvements had incurred delays to the completion of the second prototype.
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.Bowyer 1966, pp. 3–4. On 28 April 1937, an initial production order for 87 aircraft was received by Boulton Paul for the P.82; as this was prior to the first flight of the prototype, the aircraft had effectively been ordered 'off the drawing board'.Bowyer 1966, p. 4. The order for the rival Hotspur was cancelled in 1938.
Completing its acceptance tests with the turret installed, the Defiant attained a top speed of and subsequently was declared the victor of the turret fighter competition. Flight trials had revealed the aircraft to possess positive flight characteristics and considerable stability, which was of particular value when using the turret. According to aviation author Michael Bowyer, the usefulness of the Defiant had suffered due to the overly long development time for the type, observing that the Defiant's service entry was delayed to such an extent that only three production aircraft had reached the RAF, and these were only for trial purposes, by the outbreak of the Second World War. Due to delays with the type entering production, there were not enough available Defiants to begin standing patrols in 1940, by which point the introduction of not only more advanced fighters but bombers as well had allegedly undermined the usefulness of the type.
Production 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. Apart from some detail changes, the production Defiant Mk I looked similar to the two Defiant prototypes. It was powered by the Rolls-Royce Merlin III engine, which was capable of generating 1,030 hp/768 kW or 1,160 hp/865 kW.
By January 1940, over half of the original production batch had been completed. Beyond the initial production order in April 1937, follow-on orders had been issued for the type; in February 1938, an additional 202 Defiant Mk I aircraft were ordered; three months later, another 161 aircraft were ordered. At the same time Boulton Paul were engaged in production of the Blackburn Roc.Brew p 36 In December 1939, yet another 150 aircraft were ordered, raising the overall total to 513. In 1940, this rose to 563 Defiant Mk Is on order, while a further 280 were ordered under a rearranged manufacturing plan issued in mid-1940. However, the performance of the Defiant had been determined to be inadequate by this point, which led to manufacturing being sustained principally for economic reasons. A total of 713 Defiant Mk I aircraft were completed.
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. On 20 July 1940, N1550, the first production Defiant Mk II performed its initial flight. The Mk II featured a pressurised fuel system, additional fuel, an enlarged rudder, a deeper radiator, a modified engine mounting and elongated cowling. Once sufficient numbers of the Merlin XX engine were available, production of the improved variant commenced; in August 1941, the first production deliveries of the Defiant Mk II took place.