Arming a fighter plane with nothing more than a four-gun turret proved unique—but not brilliantly successful.

The German pilot could scarcely believe his luck as he approached a British fighter west of Dunkirk on May 27, 1940. The Englander, apparently a Hawker Hurricane, was taking no evasive action as the Messerschmitt Me-109E fighter closed in from behind. 

Suddenly and unexpectedly, four .303-caliber Browning machine guns blazed from a turret in the British fighter’s fuselage. Mortally hit, the 109 fell victim to Squadron Leader Philip A. Hunter of No. 264 Squadron, Royal Air Force (RAF), and his gunner, Sergeant F.H. King—and to an unusual addition to the RAF arsenal, the Boulton Paul Defiant.

Over the next few days, more Messerschmitts would suffer a similar fate before they got the measure of the new British fighter—but when they did, the encounters would have a very different outcome.

The story of the Defiant actually began during the previous war, when the British were enjoying great success with a two-seat fighter, the Bristol F.2B. What seems to have been forgotten by the RAF soon after World War I, however, was that the Bristols gained a formidable reputation by being flown like single-seaters, using the front gun as their principal offensive weapon and the gunner only for rear defense.

The RAF replaced its aging Bristols in 1931 with the Hawker Demon, a fighter version of Hawker’s versatile Hart two-seat day bomber, along with a shipboard variant, the Osprey. The rear gunner’s cockpit on the Demon was equipped with the same .303-caliber Lewis machine gun and manually operated Scarff mounting ring used on World War I aircraft with half the Demon’s 200-mph speed. The gunners had difficulty clearing jams and changing ammunition drums with freezing fingers, plus the force of the slipstream made among the gun difficult.

A later version of the Demon was fitted with a Frazier-Nash gun mount with power-assisted traverse and a windscreen for the gunner. Thus equipped, the Demon’s gunnery scores went up dramatically, but the gunner was still exposed to the elements, and his gun still had to be manually elevated and aimed. In addition, the single Lewis gun was proving inadequate against a new generation of faster and more heavily armed fighters.

One aviation firm closely concerned with the problem was Boulton Paul Aircraft Ltd., the company that had been subcontracted by Hawker to build most of the Demons for the RAF. Originally a woodworking concern based in Norwich, England, Boulton and Paul Ltd. (as it was originally called) built Camel fighters during World War I under license from Sopwith, the predecessor of Hawker. In addition to the Demon, Boulton Paul in the 1930s was producing twin-engined bomber for the RAF called the Sidestrand, and its gunners were experiencing difficulties similar to those reported by gunners of the two-seaters.

Boulton Paul developed its own pneumatically powered gun turret in 1933 for an improved version of the Sidestrand bomber called the Overstrand. Boulton Paul staff members were then approached by the French company Society d’Applications des Machines Motrices (SAMM), which offered production rights to a gun turret designed by one of its engineers, J.B.A. de Boysson.

No. 264 Squadron was the first to be equipped with the Defiant, taking it into combat through the early days of the Battle of Britain. It became a night fighter unit after disastrous losses during daylight operations and converted to de Havilland Mosquito Mk. IIs in May 1942. (IWM CH873)
No. 264 Squadron was the first to be equipped with the Defiant, taking it into combat through the early days of the Battle of Britain. It became a night fighter unit after disastrous losses during daylight operations and converted to de Havilland Mosquito Mk. IIs in May 1942. (IWM CH873)

The French Armee de l’Air had shown no interest in the idea of a power-operated gun turret. SAMM therefore was eager to recoup some of the development cost of the project by selling it to the British. A fully enclosed, electrohydraulically operated unit capable of incorporating either four machine guns or a 20mm cannon, the French turret was capable of a 360-degree traverse and elevation up to 85-degrees above the horizon by means of a single control stick.

Boulton Paul installed a four-gun de Boysson turret in the nose of an Overstrand bomber to demonstrate it to the Air Ministry. The next logical step was to develop a two-seat fighter equipped with the turret. The now aging Hawker Demon was too small and underpowered to accommodate the heavy turret, so an entirely new aircraft had to be designed. The Air Ministry issued a specification in April 1935 outlining the design parameters for the new plane. The primary mission of the plane was to attack incoming bomber formations with its four-gun turret, either from below or from the side. It was also originally to have been equipped with light bomb racks for a secondary ground attack role, but that requirement was later dropped. Since the turret fighter was expected to attack in conjunction with the new Hawker Hurricane, its performance was expected approximate that of a conventional single-seat fighter.

Royal Air Force pilots felt that the turret fighter would stand little chance against a modern, single-seat fighter, but it was never intended to encounter such opposition. The nearest prospective enemy bombers were expected to have to fly all the way from Germany, far beyond the range of escorting fighters.

At that stage, the Air Ministry issued the fateful decree that the new fighter’s armament should be restricted solely to the turret’s .303 Browning machine guns. Part of the reason was a desire to save weight. There was also a prevailing belief that the turret constituted the aircraft’s principal armament, and the pilot should be compelled to regard it as such. It was feared that, should the pilot be provided with forward firing armament, he would be tempted to use the aircraft in a conventional manner, and thus cancel out the turret’s supposed advantage. Essentially, the pilot became a chauffeur for his gunner—a role that probably did not go down well with aggressive fighter pilots.

A gunner of No. 264 Squadron prepares to enter the turret of his Defiant. He is wearing a GQ Parasuit, supplied exclusively to Defiant gunners, which incorporated a parachute harness and life-saving jacket. (IWM CH874)
A gunner of No. 264 Squadron prepares to enter the turret of his Defiant. He is wearing a GQ Parasuit, supplied exclusively to Defiant gunners, which incorporated a parachute harness and life-saving jacket. (IWM CH874)

Boulton Paul’s new fighter, called the Defiant, superficially resembled the Hurricane and Spitfire. But it was in no sense a copy of either. Designed by J.D. North, the Defiant was an all-metal, low-wing monoplane with a monocoque fuselage and retractable landing gear. It was powered by the same 1,030-hp Rolls-Royce Merlin III liquid-cooled engine used by its single-seat stablemates. The radiator was mounted under the center section of the wing, as on the Hurricane.

The Defiant achieved a reasonably high speed despite its bulky turret because of the careful attention Boulton Paul had paid to streamlining. Retractable fairings, hydraulically operated, were installed behind and in front of the turret to smooth the airflow over the fuselage when the turret was not in use.

The Defiant pilot’s cockpit was provided with a sliding canopy, but the gunner entered his turret through an automobile-type door in the fuselage side. The RAF considered that a potentially hazardous arrangement, so the turret was redesigned with an access hatch in the roof, plus an emergency escape hatch in the floor. The electrically fired turret guns were provided with an interrupter mechanism to prevent the gunner from accidentally shooting his own aircraft. Likewise, to keep the radio antenna out of the line of fire, the Defiant’s designers had to install it below the fuselage. They devised a method of automatically retracting that vulnerable equipment into the plane’s belly whenever the landing gear was lowered.

The Defiant was quite compact for a two-seat fighter—only 3 1/2 feet longer than the Hurricane and with nearly a foot less wingspan. It weighed 6,078 pounds empty and had a gross weight of 8,600 pounds, compared with the Hurricane’s empty and gross weights of 4,743 and 6,218 pounds, respectively. That extra weight resulted in a top speed of 302 mph at 16,500 feet, compared to 316 mph for the Hurricane. It also took the Defiant 11.4 minutes to reach that altitude, compared to only 6 1/2 minutes for the Hurricane.

The Defiant prototype was first flown on August 11, 1937, but the first squadron to be equipped with the new fighters, No. 264, was not deployed until December 1939. The reasoner the delay was that the Defiant was developed during a period when Boulton Paul was in the process of a corporate reorganization and was also relocating from Norwich to a new plant in Wolverhampton.

Despite the Defiant’s protracted development, that of its main rival, the Hawker Hotspur, had been even slower, due to Hawker’s preoccupation with production of its new Hurricane. Production of the Hotspur would have depended on Boulton Paul in any case, since the latter owned the sole British merchandising rights to the four-gun power turret.

The Defiant’s first saw action with No. 264 Squadron against the Luftwaffe on May 12, 1940, downing two Junkers Ju-88As. The next day, however, the Defiant’s ran into their first Bf-109Es, and though they downed five, they lost an equal number of their own planes. 

In subsequent fighting over Dunkirk in late May, the Defiant’s initially presented a shock to German fighter pilots who, thinking they were attacking a Hurricane from the rear, suddenly found themselves on the wrong end of a quartet of Brownings. During that period, Defiants piloted by Squadron Leader P.A. Hunter and Nicholas G. Cooke were credited in downing nine enemy aircraft each. Frederick Desmond Hughes and Eric Gordon Barwell (a pilot and gunner team) accounted for seven. Once Luftwaffe fighter pilots became familiar with the Defiants, they quickly mastered them, and No. 264 Squadron lost 14 aircraft during the last three weeks of May.

The second Defiant squadron, No. 141, fared even worse. In the squadron’s first engagement on June 28, nine of its Defiants tangled with a group of Bf-109Es and lost seven of their number in exchange for only four Messerschmitts. On July 19, nine more Defiants of No. 141 encountered Bf-109Es of Jagdgeschwader 51, and seven more were lost, only one of the enemy being claimed in return by one of the two surviving teams, Flt. Lt. Hugh N. Tamblyn and Sergeant S.W.H. Powell. In order to mitigate further losses, the two Defiant squadrons were pulled back to a sector farther north in England.

With the onset of the Battle of Britain in August 1940, the RAF found itself in desperate need of every fighter it could muster, and the Defiant squadrons found themselves heavily engaged once more. A third unit of Defiants, manned by expatriate Poles, No. 307 Squadron, was organized in September 1940 but did not become operational until September.

After the Battle of Britain reached its climax in September, the Luftwaffe was forced to switch to night-bombing attacks. During. that period, the Defiants were redeployed exclusively as night fighters. By May 1941, RAF Fighter Command had seven operational squadrons of Defiants, and in spite of an initial lack of on-board radar, they were the most successful night fighters in British service during 1941.

The Defiant Mk.II, equipped with A.I. Mark.VI radar and a more powerful Merlin XX engine, performed admirably as a night fighter. (IWM ATP9780B)
The Defiant Mk.II, equipped with A.I. Mark.VI radar and a more powerful Merlin XX engine, performed admirably as a night fighter. (IWM ATP9780B)

An improved version, with built-in radar, was developed specifically for the nocturnal role late in 1940. Designated the Defiant Mark II, it had a 1,260-hp Merlin XX engine that delivered 22 percent more power than the old Merlin III and increased its speed to 315 mph. Because the Defiant II gunner was restricted to the cramped confines of his turret, it was the pilot who was forced to divide his attention between the radar and his normal flight instruments. 

Higher-performance night fighters, such as the Bristol Beaufighter and the de Havilland Mosquito, became available, and the Defiant was gradually phased out of front-line service. The last ones were replaced in July 1942. A total of 1,060 Defiants were produced, including 140 of a final version, the Defiant III, built specifically as a target tug.

Boulton Paul did not immediately give up on fighter resign, despite the poor showing of the Defiant. During the desperate summer of 1940, the company submitted a proposal for a single-seat version of the Defiant Mk.II that would be armed with either 12 .303-caliber machine guns or four machine guns and four 20mm cannons in the wings. The company also designed an improved Defiant radar-equipped night fighter called the P-96, which was to be armed with either six wing-mounted cannons or a four-gun turret. The P-96 was to be powered by a 2,200-hp Napier Sabre or a 2,500-hp Bristol Centaurus engine, and was expected to attain a speed of 410 mph. Neither of those proposed aircraft was built.

A carrier-based version of the Defiant had originally been planned as a replacement for the Hawker Osprey. However, the Royal Navy later decided that it would be easier to graft Boulton Paul’s turret onto the net Blackburn Skua fighter/dive bomber. Because of Blackburn’s preoccupation with the production of Skuas for the Navy and Botha torpedo planes for the RAF Coastal Command, all 136 production Rocs, as the turret fighters were called, were built under subcontract by Boulton Paul.

The Defiant's heavy turret met with little success when it was fitted to the already-underpowered Blackburn Roc. (Fleet Air Arm Museum)
The Defiant's heavy turret met with little success when it was fitted to the already-underpowered Blackburn Roc. (Fleet Air Arm Museum)

First flown on December 23, 1936, the Roc weighed 4 percent more than the Defiant, while its 890-hp Bristol Perseus radial engine delivered 13.5 percent less than the Merlin III. The turret installation was not as aerodynamically clean as the Defiant’s because the Roc had not been designed as a turret fighter. With a top speed of under 225 mph, the ROC was deemed incapable of intercepting anything faster than a flying boat.

A few Rocs were briefly deployed with land-based fighter squadrons alongside the more conventional Skuas, but no squadron was ever equipped entirely with Rocs; nor were they ever deployed overseas or aboard carriers. The majority of Rocs ended their days as air-sea-rescue patrol planes or target tugs. The most action they saw was at Gosport. There, four grounded Rocs were dispersed around the naval air station with their turrets permanently manned as anti-aircraft batteries.

Hindsight is always 20-20, and the fallacy of the concept of fighters picking off enemy bombers with broadsides from their multigun turrets now seems obvious. It should be remembered, however, that when the Defiant was designed, those bombers were known to be armed with only three puny 7.7mm machine guns. It never occurred to the RAF planners who conceived the Defiant that France and the Low Countries would capitulate in a matter of weeks, enabling the German Luftwaffe to deploy single-seat escort fighters of the caliber of the Me-109E within range of Britain. 

Boulton Paul’s powered turrets proved to be far more successful when installed as defensive armament in bombers than they had been when used offensively in fighters. They were more widely used during the war in such famous aircraft as the Lockheed Hudson, Handley Page Halifax and Avro Lancaster.

This feature originally appeared in the September 1996 issue of Aviation History. To subscribe, click here!

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