The Boulton & Paul P.70 was a design for a bomber, based on the earlier P.64 mail carrier and P.69 bomber-transport design, and was the first Boulton & Paul design to be produced with power operated gun turrets from the start.
The P.69 was related to the P.64 mailplane, the first Boulton & Paul design for some time to reach the prototype stage. The P.64 was a rather stubby looking biplane, with a streamlined fuselage, two-man crew sitting side by side in an enclosed cockpit, a fuselage that filled the gap between the wings, and two Bristol Pegasus engines mounted below the upper wings. The P.64 had a rather chequered career. It made its maiden flight successfully on 27 March 1933, but then crashed into a cricket pitch (oddly placed on the airfield) at the start of an attempted second flight on the same day. The aircraft was repaired and modified, and went to Martlesham Heath for tests. It reached an impressive speed of 185mph, but was written off after a crash during its third flight at Martlesham on 21 October 1933. Unsurprisingly no orders were placed for the type. The design was then modified to fill specification C.26/31, for a bomber/ troop transport.
The P.70 was produced in response to Specification B.9/32. Although the overall design was outdated, losing out to monoplanes, it did feature two powered turrets - a nose turret based on the model then being developed for the Sidestrand and installed on the P.75 Overstrand, and a retractable dorsal turret.
The nose turret was a glazed cylinder with vertical sides and a domed top and bottom. On the Overstrand the top of the turret was higher than the fuselage, giving it a rather old fashioned look, but on the P.70 the fuselage filled the entire gap between the upper and lower wings, so the turret would have been faired in at the top and bottom. The tall fuselage also meant that there was space for the retractable dorsal turret, which could be lowered to reduce drag. The P.70 had a recessed bomb bay, with the forward part below the pilot's cockpit and the rear below the navigation position. The rear gunner also operated as the ventral gunner and radio operator, with his radio position between the rear gun and the dorsal turret.
Boulton & Paul's design wasn't given a prototype order, but the specification did result in the Handley Page Hampden and the Vickers Wellington, both much more advanced monoplane designs.