Boulton Paul Defiant

The Boulton Paul Defiant was one of the most unusual fighters of the Second World War, a British aircraft with all its armament, four machine guns, mounted in a powered turret behind the pilot, and no guns firing forward. Designed to attack bombers from below and from the side, the Defiant enjoyed early success in the Battle of Britain, but German fighter pilots quickly learned to attack it from the front or from below, where it was defenceless, and the aircraft was withdrawn from daylight fighting and rebuilt as a night fighter.

1. Key Facts

Field	Value
Type	Two-seat turret fighter (later night fighter)
Country	United Kingdom
Service period	1939–1945 (day fighter to 1940; night fighter to 1942)
Production	1,064 built
Length	10.77 metres (35.3 feet)
Wingspan	11.99 metres (39.3 feet)
Height	3.53 metres (11.6 feet)
Crew	2 (pilot and air gunner)
Engine	Rolls-Royce Merlin III, 1,030 horsepower
Speed	489 km/h (304 mph) at 5,200 metres
Range	About 750 km (466 miles)
Service ceiling	9,250 metres (30,350 feet)
Main armament	4 × .303-inch Browning machine guns in a powered turret
Bomb load	None (fighter only)
Main theatres	Battle of Britain; night defence of Britain; training

2. Development and Design

The Turret Fighter Concept

The Boulton Paul Defiant was based on a concept that had been popular in the 1930s, the turret fighter, which was intended to attack bombers from any direction, with a gunner in a powered turret who could aim independently of the pilot. The idea was that the fighter would fly alongside or below a bomber formation, and the gunner would fire at the bombers while the pilot concentrated on flying, a division of labour that was thought to be more effective than the fixed forward-firing guns of conventional fighters. The Air Ministry issued Specification F.9/35 for a two-seat turret fighter, and Boulton Paul, which had experience with turrets, won the contract with a design that used the Rolls-Royce Merlin engine and a four-gun turret designed by the French company SAMM.

A Design Built Around the Turret

The Defiant was designed around its turret, which was placed behind the pilot and which could rotate 360 degrees, although the guns could not fire directly forward, because the propeller was in the way, and the turret had to be traversed to the side or to the rear to fire. The turret was powered hydraulically and was operated by the gunner, who sat in the turret and who had a good view in all directions except forward. The pilot had no guns of his own, and he relied entirely on the gunner to shoot down enemy aircraft, a radical departure from fighter design that would prove to be the aircraft’s fatal flaw. The prototype first flew in August 1937, and it was ordered into production in 1938, with the first aircraft delivered to the RAF in December 1939.

The Merlin Engine and Performance

The Defiant was powered by a Rolls-Royce Merlin III engine, the same engine that powered the Hurricane and the Spitfire, producing 1,030 horsepower and giving the aircraft a top speed of 489 kilometres per hour at 5,200 metres. The aircraft was heavier than a Hurricane, however, because of the turret and the second crew member, and it was less manoeuvrable, with a slower rate of climb and a wider turning circle. The Defiant was also less well armed in practice than its four guns suggested, for the guns could only fire to the side and to the rear, and the aircraft had to manoeuvre to bring them to bear, which was difficult against a fast-moving fighter.

3. Technical Specifications

The Four-Gun Turret

The turret of the Defiant was its most distinctive feature, a powered, hydraulically operated turret mounting four .303-inch Browning machine guns, with a combined rate of fire of about 4,800 rounds per minute. The turret could rotate 360 degrees, but the guns were limited in elevation and depression, and they could not fire forward through the propeller arc, which meant that the aircraft was defenceless against an attack from the front. The gunner sat in the turret, which was cramped and uncomfortable, and he had to operate the guns, the turret traverse, and the radio, while also watching for enemy aircraft. The turret was also heavy, and it raised the centre of gravity of the aircraft, which affected its handling.

The Merlin III and Flight Performance

The Merlin III engine gave the Defiant a top speed of 489 kilometres per hour, a range of about 750 kilometres, and a service ceiling of 9,250 metres, which was adequate for a fighter of the period, although it was slower than the Hurricane and the Spitfire. The aircraft had a retractable undercarriage, an enclosed cockpit for the pilot, and a radio, and it was fitted with a reflector gunsight for the gunner, although the gunsight was of limited use in the turret. The Defiant was stable and easy to fly, but it was not agile, and it could not compete with single-seat fighters in a dogfight.

Crew, Armour, and Equipment

The Defiant had a crew of two, a pilot and an air gunner, who worked as a team, with the pilot flying the aircraft and the gunner operating the turret. The aircraft had some armour protection for the pilot, but the gunner was exposed in the turret, and the aircraft had no self-sealing fuel tanks in its early versions, which made it vulnerable to fire. The Defiant was also fitted with a radio and with navigation equipment, and later versions were fitted with airborne interception radar for night fighting, which allowed the aircraft to find enemy bombers in the dark.

4. Combat Service

Early Success in the Battle of Britain

The Defiant first saw combat in May 1940, during the evacuation from Dunkirk, where No. 264 Squadron claimed to have shot down 37 German aircraft in a single day, although this figure was later found to be exaggerated. The Defiant’s early success was due to the fact that German pilots mistook it for a Hurricane and attacked it from behind, where the turret guns could fire at them, and the aircraft’s four guns were effective against bombers and against fighters that approached from the rear. The success was short-lived, however, for German pilots quickly learned to recognise the Defiant and to attack it from the front or from below, where it had no guns, and the aircraft suffered heavy losses.

The Disaster of 19 July 1940

The turning point for the Defiant came on 19 July 1940, when No. 141 Squadron, flying Defiants, was attacked by Bf 109s off the south coast of England, and six of nine aircraft were shot down in a few minutes, with the loss of most of their crews. The disaster showed that the Defiant was no match for single-seat fighters, and the aircraft was withdrawn from daylight operations, although it continued to fly in the Battle of Britain as a day fighter for a few more weeks, with mixed results. By the end of August 1940, the Defiant had been withdrawn from day fighting, and it was clear that the turret fighter concept had failed.

The Night Fighter Role

After its withdrawal from day fighting, the Defiant was rebuilt as a night fighter, fitted with airborne interception radar and with a black camouflage scheme, and it served in the night defence of Britain during the Blitz of 1940–1941. As a night fighter, the Defiant was more successful, for it could approach bombers from below and from the side, where its turret guns were effective, and it shot down a number of German bombers, including Heinkel He 111s and Dornier Do 17s. The Defiant was, however, soon replaced by more capable night fighters, such as the Bristol Beaufighter and the de Havilland Mosquito, which had radar and heavier armament, and by 1942, the Defiant had been withdrawn from front-line service.

5. Variants

The Defiant Mk I and Mk II

The Defiant was produced in two main versions, the Mk I, with a Merlin III engine, and the Mk II, with a more powerful Merlin XX engine, which gave slightly better performance and which was fitted with radar for night fighting. The Mk II also had a larger fuel capacity and improved equipment, and it was the version that served as a night fighter. There was also a Defiant TT, or target tug, version, which was used to tow targets for gunnery training, and which served until the end of the war.

The Night Fighter and Target Tug

The night fighter Defiants were fitted with AI Mk IV radar, which allowed them to detect bombers at night, and with a black finish that made them difficult to see. The target tug Defiants were stripped of their turrets and fitted with a winch for towing targets, and they were used by training units to teach gunners how to shoot. The Defiant also served as a test aircraft for new equipment, including radar and ejection seats, and it was used by the Royal Navy for trials.

Unbuilt Developments

Boulton Paul proposed several developments of the Defiant, including a version with a more powerful engine and with forward-firing guns, but none was built, and the company turned to other designs, including the Balliol trainer. The Defiant remains the only turret fighter to see combat in the Second World War, and its failure marked the end of the turret fighter concept. One proposal fitted the Defiant airframe with a Merlin XX engine and two forward-firing cannon, but the Air Ministry judged that the Hurricane and Spitfire already filled that role, and the design was never built.

6. Strengths

Effectiveness Against Bombers

The great strength of the Defiant was its effectiveness against bombers, for its turret guns could fire at a bomber from below or from the side, where the bomber’s defences were weakest, and the aircraft could stay with a bomber formation and fire continuously. In its early operations, the Defiant shot down a number of German bombers, and it was feared by bomber crews, who did not expect to be attacked from below. The aircraft was also effective as a night fighter, where its turret and its radar allowed it to find and destroy bombers in the dark.

A Stable Gun Platform

The Defiant was a stable gun platform, with a steady flight and a good field of fire for the gunner, and it was easy to fly, which allowed the pilot to concentrate on positioning the aircraft while the gunner aimed the guns. The aircraft was also well built and reliable, with a proven Merlin engine and a simple structure, and it could operate from the same airfields as Hurricanes and Spitfires. The turret could be traversed smoothly in flight, and the gunner had a clear view to the sides and to the rear, which allowed him to keep a bomber in his sights while the pilot held the aircraft steady on its course.

The Turret Concept

The turret concept itself had some merit, for it allowed the guns to be aimed independently of the aircraft, and it gave the gunner a wide field of fire. The idea was later used in bomber turrets and in the turrets of American fighters such as the Bell P-39 Airacobra, although it was never again used as the sole armament of a fighter. In level flight the Defiant was steady and predictable, and the gunner could track a bomber through a wide arc without the pilot having to manoeuvre violently, a quality that made the aircraft effective when it was able to approach a bomber formation unmolested. The Defiant showed that a turret could be effective, but only if the aircraft also had forward-firing guns.

7. Weaknesses

No Forward-Firing Guns

The main weakness of the Defiant was its lack of forward-firing guns, which meant that it was defenceless against an attack from the front, and that it could not attack an enemy aircraft that was ahead of it. This was a fatal flaw in a fighter, for most air combat involved head-on attacks and attacks from the front quarter, and the Defiant could not respond to them. German pilots quickly learned this, and they attacked Defiants from the front, where the turret could not fire, and shot them down with ease.

Weight, Manoeuvrability, and Vulnerability

The Defiant was heavy and unmanoeuvrable, with a slower rate of climb and a wider turning circle than single-seat fighters, and it could not escape from a Bf 109 once it was attacked. The turret and the second crew member added weight and drag, which reduced performance, and the aircraft was vulnerable to cannon fire, with no armour for the gunner and no self-sealing tanks in early versions. The Defiant was a brave attempt at a new idea, but it was a dead end in fighter design.

The Failure of the Turret Fighter Concept

The failure of the Defiant showed that the turret fighter concept was flawed, for a fighter needs forward-firing guns to attack and to defend itself, and a turret alone is not enough. The concept was based on the assumption that bombers would be the main target, and that fighters would not attack the turret fighter, but in practice, German fighters did attack, and the Defiant could not defend itself. The lesson was learned, and no turret fighter was ever built again.

8. Comparison with Rivals

Against the Hawker Hurricane and Supermarine Spitfire

Against the Hurricane and the Spitfire, the Defiant was slower, less manoeuvrable, and less well armed in practice, although it had the same engine as the Hurricane. The Hurricane, with eight forward-firing machine guns, could attack from any direction, while the Defiant could only fire to the side and to the rear, and the Spitfire, with its speed and agility, was in a different league. The Defiant was withdrawn from day fighting as soon as Hurricanes and Spitfires became available in sufficient numbers, and it never regained its place.

Against the Messerschmitt Bf 109

Against the Bf 109, the Defiant was outmatched in every respect, with the German fighter faster, more manoeuvrable, and better armed, with cannon as well as machine guns. The Bf 109 pilots learned to attack Defiants from the front, and they shot them down with ease, as the disaster of 19 July 1940 showed. The comparison is a stark illustration of the failure of the turret fighter concept, and of the importance of forward-firing guns in a fighter.

Against the Bristol Beaufighter

Against the Beaufighter, which replaced it as a night fighter, the Defiant was slower, less well armed, and less well equipped with radar, and the Beaufighter, with its heavy cannon armament and its powerful engines, was a far more capable night fighter. The Defiant was a stopgap night fighter, and it was replaced as soon as a better aircraft became available. The Beaufighter carried four 20 mm cannon and six machine guns, had two powerful Hercules engines, and could stay on patrol far longer than the Defiant, which made it the superior night fighter in every respect.

9. Legacy

A Brave Failure

The legacy of the Boulton Paul Defiant is its status as a brave failure, an aircraft that tried a new idea and that failed, but that did so with courage and with some success. The Defiant is remembered for its early victories in the Battle of Britain, and for the men who flew it, who knew its weaknesses and who fought on regardless. The aircraft is preserved in the Royal Air Force Museum at Hendon, where visitors can see the only surviving Defiant, and it is remembered as a reminder that innovation in warfare is risky, and that not every new idea succeeds.

Lessons for Fighter Design

For aircraft designers, the Defiant is a lesson in the importance of forward-firing guns and of manoeuvrability in a fighter, and in the dangers of designing an aircraft around a single concept without considering how it will be used in combat. The Defiant also shows the importance of learning from combat, for the RAF quickly recognised the aircraft’s weaknesses and withdrew it from day fighting, and it found a useful role for it as a night fighter and as a target tug. The Defiant deserves its place in history as a unique and courageous aircraft, even if it was not a successful one.

Remembering the Defiant Crews

The crews of the Defiant are remembered for their skill and their bravery, and for the battles they fought in the summer of 1940, when they faced the Luftwaffe in an aircraft that was outmatched and outgunned. The pilots and gunners of the Defiant, many of whom were lost, deserve to be remembered alongside the pilots of the Hurricane and the Spitfire, for they too fought in the Battle of Britain, and they too paid the price. No. 264 Squadron, the first Defiant squadron, lost many of its pilots and gunners in 1940, yet its survivors continued to fly the aircraft at night, and their service is commemorated at the Royal Air Force Museum and in the squadron’s records.

Frequently Asked Questions

How many Boulton Paul Defiants were built?
1,064 Defiants were built, including day fighters, night fighters, and target tugs, and the aircraft served with the RAF from 1939 to 1945.

Why did the Defiant have no forward-firing guns?
The Defiant was designed as a turret fighter, with all its guns in a powered turret behind the pilot, on the theory that the gunner could attack bombers from any direction while the pilot flew the aircraft.

How successful was the Defiant in the Battle of Britain?
The Defiant had early success against bombers, but German fighter pilots learned to attack it from the front, where it was defenceless, and it suffered heavy losses, including six aircraft lost on 19 July 1940.

What happened to the Defiant after the Battle of Britain?
The Defiant was withdrawn from day fighting and rebuilt as a night fighter, where it was more successful, shooting down German bombers during the Blitz, before being replaced by the Beaufighter and the Mosquito.

Did the Defiant serve as a night fighter?
Yes, Defiant night fighters, fitted with radar, served in the night defence of Britain in 1940–1942, and they shot down a number of German bombers.

Are any Defiants still in existence?
Yes, one Defiant survives, on display at the Royal Air Force Museum in Hendon, London, where it is preserved as a reminder of the turret fighter concept.

Sources

• Alec Brew, The Turret Fighters: Defiant and Roc (Crowood Press, 2002)
• Philip Moyes, The Boulton Paul Defiant (Profile Publications, 1966)
• The Royal Air Force Museum, Hendon, Defiant collection
• Francis K. Mason, The British Fighter Since 1912 (1992)
• The Imperial War Museum, Battle of Britain collection