The Boulton Paul P.75 Overstrand was the first RAF aircraft to have an enclosed powered gun turret, and was developed from the earlier Boulton & Paul Sidestrand. It was also the last biplane bomber to enter service with the RAF.
The Sidestrand was a three bay biplane that was produced in prototype form in 1925, made its maiden flight in 1926 and was ordered into small scale production. The Sidestrand could reach 140mph, and it soon became obvious that this was too fast for an open nose gun position. Not only was it very difficult to actually aim the gun in 140mph wind, on several occasions the gunners dropped ammo magazines while trying to reload the gun, damaging the propellers.
On 13 August 1932 the Air Ministry asked Boulton & Paul to work out some way to protect the front gunner from the slip stream. At first the company looked at the idea of fitting a windshield to the gun position, but this idea was soon discounted. On 28 December 1932 the Air Ministry wrote to Boulton & Paul to give more details of their requirements. Protection was to be provided in every firing position, but the field of fire shouldn't be reduced. It was clear that this would require an enclosed gunner's position that would move with the gun.
Boulton & Paul were given a contract to produce the new turret and install it on an existing Sidestrand. The limited amount of space available on the Sidestrand meant that the bomb aiming position would also have to be carried within the turret. The new turret was designed by Boulton & Paul's Armaments Section, headed by H A 'Pop' Hughes.
The turret took the form of a glazed cylinder, with vertical sides and a domed top and bottom. It was powered by compressed air which turned a spindle mount at the base of the turret. There was a large entry door at the back, and a long vertical gap in the front to allow for vertical movement of the gun. Compressed air was provided directly to the turret from storage bottles, which could provide enough power to rotate the turret twenty times, and could be recharged using a compressor powered by one of the engines. The turret could rotate through 360 degrees if the gun was above 70 degrees, but had a safety mechanism that stopped it rotating too far back if the gun was lower. Although the turret now looks quite primitive, it had the widest field of fire of any powered gun turret ever built, capable of covering two thirds of the sky.
The turret was mounted on a geared spindle that was carried on an extended bottom longeron. The top of the horizontal part of the turret was surrounded by a loop that was connected to the upper longeron to keep the turret in place.
When not in use the gun could be stowed at the top of the turret, and a cover zipped shut to close the gap. In use the zip was opened and closed by the movement of the gun. This was replaced on production aircraft with a simple canvas strip. The bomb aiming equipment was in the left side of the turret, with an openable bomb aiming window. The gunner had a seat in the base of the turret, which moved up or down automatically to match the movement of the gun.
The turret wasn't the only change made to the prototype. The main cockpit was also given an enclosed cockpit, with a sliding windscreen for the pilot. The rear gunner's position remained open, although a windscreen was placed just in front of it. The rear gunner had a dorsal gun and a ventral gun. All of the crew positions were heated. The outer wings were swept back to keep the aircraft balanced with the extra weight in the nose. The fuselage was strengthened, allowing the bomb load to rise to 1,500lb. Two 500lb bombs could be carried with the recessed bomb cell, and two 250lb bombs on external carriers.
The first prototype was built around Sidestrand III (J9186). It made its maiden flight in 1933. This was a very short flight, aborted after smoke came out of the new heating system, but in general tests with the new aircraft went well. It went to Bircham Newton on 22 February 1934, for trials with No.101 Squadron, the only squadron to use the Sidestrand.
Three further Sidestrand IIIs were converted to the new Sidestrand V standard, although with a slightly large diameter turret and Pegasus IIM3 engines. In March 1934 the modified design was renamed as the Overstrand (taking its name from a village near Cromer). Soon after this Boulton & Paul sold off their aircraft division, which on 30 June 1934 became the independent Boulton Paul Aircraft Ltd. This change in corporate structure was reflected in the aircraft name, which went from Boulton & Paul to Boulton Paul. This came after a prolonged period in which few Boulton & Paul designs had even reached the prototype stage, but also just before the aircraft division began to make real money from powered gun turrets.
Twenty four new Overstrands were ordered, 19 in a first batch in 1934 and five in 1935 as replacement aircraft. No.101 Squadron was the first to use the type, getting two converted Sidestrands in January-February 1935. Delivery of the production Overstrands didn't begin until early in 1936, and they had all been completed by the end of the year.
Early aircraft used the 555hp Bristol Pegasus I engine. Later production got the 58-hp Pegasus II.
Most of the Overstrands were used by No.101 Squadron, the only unit that had used the Sidestrand. The prototype arrived on 24 January 1935, and the first new aircraft in October 1935, with deliveries ending in July 1936. This made the squadron the first to go operational with an aircraft armed with a power turret, and its crest was changed to include a medieval tower. The first aircraft were used to form a new 'C' Flight, the squadron only have had two Sidestrand flights. As further aircraft arrived, they replaced the aircraft of A and B Flights. The Overstrand was a popular aircraft, combining the good points of the Sidestrand with increased comfort. The nose turret was a great success, improving the nose gunner's hit rate in tests from 15% up to 85%! The Overstrand appeared at several Hendon displays, including the display of 1937 where it demonstrated in-flight refuelling with a Vickers Virginia tanker. The squadron used the aircraft until the summer of 1938, when it was replaced by the Bristol Blenheim Mk I.
The new No.144 Squadron borrowed four Overstrands from No.101 in January 1937, operating alongside the Hawker Audax. The Overstrands were quickly replaced by Avro Ansons, and the squadron converted to the Blenheim by the end of 1937.
Some of the Overstrands were used as gunnery trainers, but the last were withdrawn by 1941. One was used to test the de Boysson four-gun turret in 1937. The last Overstrands were withdrawn after one broke up in the air over Chesil Bank in 1940 killed the entire crew.
A plan for an improved version, with retractable landing gear and an improved airframe was proposed as the P.80 Superstrand, but by this point biplane bombers were clearly obsolete.
Overstrand K8176 was used to test the impact of firing a 20mm cannon on the structure of the aircraft. A single 20mm Hispano cannon was placed on a pillar mount in an open position in the nose.
Engine: Two Bristol Pegasus IIM.3 radial engines
Span: 72ft 0in
Length: 46ft 0in
Height: 15ft 6in
Empty weight: 7,936lb
Maximum take-off weight: 12,000lb
Max speed: 153mph at 6,500ft
Service ceiling: 22,500ft
Range: 545 miles
Armament: Three 0.303in Lewis guns (nose, dorsal and ventral positions)
Bomb load: 1,600lb