On the sale of its assets and business to a public company on 30 June 1934, Boulton & Paul became Boulton Paul Aircraft Ltd. Plans were also at hand to build a new factory at Wolverhampton. A contract was received for the construction of Hawker Demons fitted with a Frazer-Nash hydraulically-operated turret, which effectively protected the gunner from the slipstream enabling him to better train and fire his gun. The rear top fuselage consisted of a turtleback cowl attached to the gunner’s harness which folded up and down according to his movements.

Boulton Paul P82 Defiant Article

Although development of powered turrets continued, it would appear that funding was somewhat limited, notwithstanding the enthusiastic response from the Air Ministry. The companys patents in this field ended in the Secret List, thus severely limiting further progress. John D. North, the company’s chief engineer, adopted a French design which met little official interest in France. Designed by J.B.A de Boysson, a hydraulically-operated turret was developed by the Societe d’Applications des Machines Motrices (SAMM) and Boulton Paul acquired full rights for its manufacture and development within the British Empire. Two turrets were ordered from SAMM for further trials, fitted with Browning machine guns in place of the four Darne 7 mm guns of the original design. Meanwhile, an original (Darne-armed) Boysson turret was tested in the nose of Overstrand K8175, and a single 20mm Hispano cannon installation was also tested in another turret on K8176.

During this time of radical change in fighter design and tactics, the Air Ministry was probably not completely confident that the emerging category of single-seat monoplane fighters, armed with a battery of eight guns buried in the wings, could make the grade. The issue of Specification E9/35 called for a two-seat day and night fighter, which should have its armament concentrated in a power-operated turret. In spite of the turret’s weight penalty, the aircraft was expected to perform within performance parameters close to those of contemporary single-seaters not only in speed but also in range of action.

Apart from Boulton Paul, F.9/35 brought proposals from Hawker, Bristol and Armstrong Whitworth; of these only the first two made it beyond the design stage, with one prototype each being ordered in the autumn of 1935. Boulton Paul commenced construction of their E9/35 prototype (K9310) in 1936 at their new Wolverhampton plant, although design had begun at Norwich. Close attention had been given to aerodynamic cleanliness in order to minimise drag. Of conventional construction, its fuselage was built in two main sections, the forward section built up of four longerons and a number of bulkheads, while the rear section was made up of three units, the two side panels and the top decking. The forward section housed the pilot’s cockpit over the wing centre section. The rear half of the fuselage incorporated the turret which was faired fore and aft with wooden-framed hydraulically-operated fairings which automatically hinged down to allow the guns to traverse.

The wings, built around two spars, were broken down into five parts: a centre section, two outer wing panels and detachable wing tips. The centre section housed the fully-retractable main undercarriage members and self-sealing fuel tanks, one 52 Imp. gallon (236 litre) tank on either side of the radiator bath. This tankage was later increased by another pair of 27 Imp. gallon (123 litre) tanks further outboard in the outer wing sections.

Its all-metal structure was conventional in most respects, its only unusual feature being the method of attaching the light alloy skinning to the stringers and ribs and then attaching these to the fuselage frames and wing spars. This obviated the need to pre-form the skins and, by riveting them while flat and countersinking the rivets, an exceptionally smooth finish was obtained.

The Boulton-Paul A. Mk.IID turret was a self-contained, removable unit, fitted with four .303 Browning machine guns with 600 rounds per gun. This unit weighed 3601b (162kg) empty, to which 881b (39.6kg) of armament and 1061b (48kg) of ammunition had to be added. Its hydraulically-operated system was completely independent from any other system in the aircraft.

The Boulton Paul Defiant was only the RAF’s third monoplane aircraft and the Defiant became the RAF’s first four machine gun fighter. On 11 August 1937, chief test pilot Cecil Feather took the turret-less prototype up for its first flight. The aircraft weighed 7,5001b (3,375kg) without its turret at this stage and was powered by a 1,030-hp (768-kW) Rolls Royce Merlin 1 engine with which it attained a maximum speed of 302mph (485.6km/h) , at which point it was named the Defiant. With the turret installed, vertical tail area had to be increased slightly; meanwhile a second prototype had been ordered (K8620).

When it had been decided to abandon further development of the Hotspur, an initial order for 87 production aircraft of what by that time had become known as the Defiant, was placed on 28 April 1937. Although drawings began reaching the workshops by the end of that same year, completion of the second prototype was delayed due to a change in power plant, from Merlin 1 to Merlin II. Apart from the increased tail area, it also featured redesigned exhaust stacks and cockpit canopy, and modifications to the undercarriage doors, thus bringing it very close to production standard. This set the first flight of K8620 back to 18 May 1939.

The second prototype was fitted with a Type A four-gun turret based on a French design already licensed for use on Boulton Paul’s Overstrand bomber, and this version with but minor changes became the production Defiant Mk1. The turret was electro-hydraulically operated with a mechanical backup and carried 4 x .303 Browning machine guns, electrically fired with cut-off points in the turret ring preventing activation when pointing at the propeller disc or tailplane. Whilst the gunner could lock the turret forward and transfer firing control to the pilot, this was rarely practised given forward elevation restrictions and the lack of pilot gunsight. The machine guns were paired two to a side and the entire turret system could scan a 360-degree area above the aircraft. Six-hundred rounds of 7.7mm ammunition were afforded each gun.

This was followed by another engine update with the selection of the Merlin III, the version with a standardised shaft for DH or Rotol constant-speed propellers, for production Defiants. The first production aircraft (L6950) performed its maiden flight on 30 July 1939, and by September it was with the A & AEE at Boscombe Down for official trials. These included dive-bombing trials at Orfordness, as L6950 had been fitted with underwing racks to carry light bombs. L6951 was transferred to the Central Flying School for handling trials. The Defiant was described as having excellent handling qualities, with very few vices. It found to be quite stable, with very little trim change being necessary when the undercarriage or flaps were extended or retracted. As with most aircraft of the time, it had a tendency to swing to port during take off, something which could easily be corrected. Comparative trials with No.111 Squadron Hurricanes were performed at Northolt on L6952 in October 1939, where the real situation emerged: the Defiant proved to be at a distinct disadvantage when compared with a single-seat fighter. The report clearly stated that than any average pilot flying a Hurricane could carve up an aircraft with the power/weight ratio of the Defiant.

The first unit to receive the Defiant was No.264 Squadron, which had been formed at Sutton Bridge in October 1939 from where it moved to Martlesham. There it received its first two aircraft on 8 December 1939. By that time, more than half of the first production order had been completed; two further orders had been placed, one in February 1938 for 202 aircraft and another in May of the same year for 161. More orders were placed in December 1939 (150), February 1940 (50) and July 1940 (280).

264 Squadron was grounded on 28 January after a series of engine difficulties and hydraulic problems, the ban being lifted during the first week of February 1940. Although not fully worked up, two of the squadron’s flights were posted to Wittering on 21 March to fly convoy patrols. On 10 May, the entire squadron moved to Duxford, while two days later a Flight proceeded to Horsharn St Faith. That day, six Defiants from this Flight patrolled the Dutch coast and after strafing ground targets, escorted by Spitfires from No.66 Squadron, drew first blood by shooting down a Ju 88A. On 13 May, B Flight went into action, engaging a formation of Ju 87Bs of which they claimed four destroyed, when the Defiants were bounced by Bf 109Es escorting the ‘Stukas’; only L6974 escaped to tell the tale. Between 27 and 31 May Deflants were ordered to participate in providing top cover at Dunkirk, across the Channel. During this period, 264 Squadron claimed 65 kills, 37 of which were supposed to have been achieved on 29 May. There is no doubt that these claims were grossly exaggerated.

No.141 Squadron was the second Defiant unit to be formed, becoming operational at Grangemouth on 3 June 1940. On 19 July, 12 aircraft from the unit moved to Hawkinge, nine of which were scrambled at 12.30hrs. They had the misfortune of being bounced south of Folkstone by Bf 109Es which practically decimated the formation; L7014, although badly shot up, was to be the sole survivor. Pilots of No.111 Squadron claimed that the Defiants had shot down four of the aggressors during the fight.

During the height of the Battle of Britain, No.264 moved to Horchurch on 22 August; two days later the unit claimed three Ju 88s and a Bf 109E destroyed for the loss of two Defiants. On 26 August three Do 17s were shot down losing another Defiant in the process, while two days later four more of the turret fighters were lost.

Following the loss by 264 Squadron of 7 aircraft with 9 crewmen dead over the three days 26th to 28th August 1940, the Defiant was withdrawn from the day fighter role. On 28 August, 264 Squadron ceased operations and moved to Duxford the following day. Offensive patrols were cancelled and Defiant sorties were limited to anti-bomber patrols over the Channel from Matlesham Heath and Debden, until on 23 July the unit was retired to Kirton-on-Lindsey.

The Defiant Night Fighter No.141 Squadron pioneered night flying sorties with the Defiant on 1 July 1940, when L7997 flew a lone sortie. As the type had been withdrawn from daylight operations, both 141 and 264 Squadrons were operating practically as night fighter units by September. This coincided with the switch to night bombing by the Luftwaffe on London. B flight of No.141 Squadron moved to Biggin Hill, with A Flight taking up residence at Gatwick in October. Two He 111s were clairned by the unit as shot down on the night of 15/16 September. No 264 was similarly engaged, flying from Rochford, and later Debden. However, successes were few and far between and it was only with the arrival or airborne radar that the situation improved.

Defiants were now the Defiant NF.Mk I and were based on the standard Mk I marks. The NF.Mk IA followed soon afterwards with the AI.Mk IV / VI interception radar.

In the quest for better performance, Boulton Paul developed the Defiant F Mk II, more dedicated to the night-fighter role with the AI Mk IV airborne interception radar and powered by a 1,260hp Merlin XX. First flight of such an aircraft (N1550) was performed on 20 July 1940. Apart from the longer cowling to accommodate the new engine, both oil cooler and radiator were deepened and enlarged. The Mk II clocked a maximum speed of 315mph (506.5km/h) at 16,500ft (5,020m) during official trials. An order for 280 Mk Is placed in July was amended to cover the production of 63 Mk Is and 210 Mk IIs (a further seven Mk Is having been converted to Mk II standard on the production line).

Deliveries of the Mk II commenced in February 1941, by which time the Defiant had established itself in the night fighter role. By autumn of that year, AI Mk IV (and later Mk VI) radar was being fitted to Mk 1 Deflants (redesignated Mk IA) and Mk IIs (NF Mk II). Night fighter squadrons with the Defiant began to mushroom, these including No 96,125, 141, 151, 153, 264 and 410. By mid1942, the Defiant night fighter had been largely replaced by twin-engined types even in this role, No 153 and 256 Squadrons retaining their turret fighters up to the end of that year. A Flight of No 515 (Special Duties) Squadron kept its Defiants well into 1943.

As Defiants became surplus to needs of front line squadrons, they were transferred to Fighter Command air-sea-rescue units. Around 50 examples were fitted with an ‘M’ type dinghy stowage pack under each wing as ASR.Mk Is, serving with No 275, 276, 277, 278 and 281 Squadrons. After just six months of operations, these Defiants were withdrawn due to a number of problems.

The final production run covered 140 TT.Mk III target towing Defiants ordered in July 1941. A clear rearsliding canopy over the winch operator’s cockpit replaced the gun turret, a ‘B’ or ‘E’ type winch driven by a windmill was fitted to the starboard side of the fuselage while the target sleeves were housed in a faired pack fitted under the rear fuselage. First TT Mk 1 (DR863), based on the Merlin XX-powered Mk II, flew in January 1942. Apart from the order for new-build machines, the last 40 F Mk IIs on the production line were also completed as TT Mk Is, with the last (AA670) being delivered in May 1942. To these one has to add a further number of Mk IIs which, on being retired frorn squadron service, were converted for target-towing duties.

Quite a number of Deflants F Mk 1 were available for conversions to TT Mk III, a job assigned to Reid & Sigrist at Desford. N3488 became the development machine (rather than prototype) and apart from the Merlin III, distinguishable by its shorter nose, this version was similar to the TT Mk 1 in most other respects. About 150 Defiants were converted to Mk III standard between 1942 and 1943. Apart from most of the Gunnery Schools, where it replaced the Lysander, also the Royal Navy employed the Defiant target-tug overseas. For this purpose, in 1943 it became necessary to fit tropical filters to enable the Defiant to operate in hot and dusty climes.

At its peak of use, 13 RAF squadrons were Defiants equipped.

The Defiant also saw service with the Royal Navy and the air forces of Australia, Canada and Poland.
A total of 1,064 machines were built.

Boulton Paul had embarked on a turret-less version way back in 1940, a single seat Defiant with a pair of .303in Brownings in each wing. The original prototype – K8310 – was modified to this standard; in fact one could say that it had reverted to the solid rear fuselage with which it had flown its initial flights. Notwithstanding that its performance would have been slightly better than that of the Hurricane, no official interest was shown in the single-seat Defiant.

At one time, a Merlin 24-powered version was under study, under the designation of TT Mk II; however the number of newbuild TT Mk Is and other conversions was considered as adequate for service needs, and the project did not proceed further.

Only one complete Boulton Paul Defiant example exists, this being held by the Royal Air Force Museum in Hendon, London.

Gallery

Defiant Mk I
Engine: one 1,030-hp (768-kW) Rolls-Royce Merlin III
Wing span: 39 ft 4 in (11.99 m)
Length: 35 ft 4 in (10.77 m)
Height: 11 ft 4 in (3.45 m)
Wing area: 250.0 sq ft (23.23 sq.m)
Empty weight: 6,078 lb (2,757 kg)
Maximum take-off: 8,600 lb (3,901 kg)
Normal loaded weight: 8318 lb / 3773 kg
Max speed: 259 mph / 302 kph at SL
Maximum speed: 304 mph (489 km/h) at 17,000 ft (5,180 m)
Cruising speed: 259 mph / 417 kph at 15,000 ft / 4572 m
Initial climb rate: 1.900 ft (579 m) per minute
Time to 15,750 ft / 4800 m: 8 min 30 sec
Service ceiling: 30,350 ft (9,250 m)
Range: 465 mi / 748 km at 259 mph / 417 kph
Armament: four 0.303-in (7.7-mm) machine guns 600 rds each
Crew: 2
Number built: 723

Defiant NF.Mk IA

Defiant TT.Mk I

Defiant Mk II
Engine: Rolls Royce Merlin XX, 1262 hp/954-kW
Length: 35.335 ft / 10.77 m
Height: 11.319 ft / 3.45 m
Wingspan: 39.337 ft / 11.99 m
Wing area: 250.048 sqft / 23.23 sq.m
Max take off weight: 8425.3 lb / 3821.0 kg
Weight empty: 6282.0 lb / 2849.0 kg
Wing load: 33.62 lb/sq.ft / 164.00 kg/sq.m
Max. speed: 272 kts / 504 km/h / 313mph
Cruising speed: 226 kts / 418 km/h
Service ceiling: 30348 ft / 9250 m
Rate-of-Climb: 1,900ft/min (579m/min)
Range: 404 nm / 748 km
Crew: 2
Armament: 4x cal.303 MG (7,7mm)
Number built: 210

Defiant NF.Mk II
Engine: Rolls-Royce Merlin XX, 1,280 hp
Top speed: 313 mph
Service ceiling: 30,350 feet
Range: 465-mile

Defiant TT.Mk III