The Boulton & Paul P.15 Bolton was the first all-metal aircraft to be delivered to the RAF, and was largely based on the earlier Boulton & Paul P.7 Bourges fighter-bomber.
Boulton & Paul began to work with metal construction on their P.10, the first practical all-metal aircraft to be produced in Britain. They would later improve their methods on the P.12 Bodmin, an experimental aircraft designed to test the possibility of having engines in the fuselage powering propellers mounted between the wings. The overall design was a failure, but it did feature Boulton & Paul's new locked joint system, which used steel tubes rolled from high quality steel strips. Despite the lower number the P.12 was delivered after the P.15, although both were produced to 1920 specifications.
The Air Ministry was also convinced that metal construction was the way to go, partly because it was expected to be more robust in British conditions than wooden construction, and partly because high quality aircraft grade spruce began to run short during the First World War. Specification 4/20 was issued to allow Boulton & Paul to continue with their work on metal aircraft. A single prototype was ordered (J6584).
The P.15 Bolton was based on the P.7 Bourges of 1918, a twin-engined aircraft that was an early example of a fighter bomber, but that didn't enter production because of the end of the First World War. The P.15 had the same layout. It was a three bay biplane, with the engines carried in nacelles mounted above the lower wings, just inside the inner struts, as seen on the third Bourges prototype (the P.7b). The wingspan of 62ft 6in was an increase of five feet over the Bourges.
The wings had two spars, each made up of four corrugated steel strips formed into a box. The interplane struts were made of two linked triangular sections.
The structure used the locked joint system, which involved steel tubes made by rolling long steel strips, which could be produced to finer tolerances than metal tubes. The fuselage had flat sides, with a triangular top, designed to allow the rear gunner to fire downwards on both sides of the aircraft. The fuselage was built around tubular longerons built in two parts to improve their strength.
The undercarriage had a narrow track than on the Bourges. A third wheel was fitting in front of the main wheels to prevent the aircraft tipping over.
The Napier Lion engines were mounted on pedestals carried on the lower wing. Each pedestal carried the radiators at the front, and the top of the engines were uncovered, to allow for air cooling. The engines were carried on anti-vibration mountings. The main fuel tanks were installed in the centre of the fuselage, between the wings. Windmill driven pumps moved fuel from the main tanks to a high mounted feeder tank, which was used to gravity feed the engines.
It carried a crew of three, each in their own open cockpit. One gunner was posted in the nose, and another just behind the trailing edge of the wing. The pilot's position was just ahead of the leading edge.
One prototype of the P.15 was built, and it became the first all-metal aircraft to be delivered to the RAF. It made its maiden flight in September 1922, but its performance figures have been lost. A lack of funds meant that no production order was placed, but the success of the design did help convince the Air Ministry that metal construction would be essential in future designs. The Bolton itself was followed by the similar Boulton & Paul P.25 Bugle, funded in 1922, and eventually by the Boulton & Paul P.29 Sidestrand, which was actually ordered into production in small numbers.
Engine: Two Napier Lion
Power: 450hp each
Span: 62ft 6in
Loaded weight: 9,500ft
Max speed: 130mph at 10,000ft (estimated)
Armament: One Lewis gun in nose, one Lewis gun in dorsal postiion