From the outset the P.B.31E needed the range and loitering times envisioned for the P.B.29E for the aircraft would have to be present prior to the arrival of German airships in British airspace. From there, the aircraft would need the firepower and performance to bring these fragile airships down before they could reconnoiter or drop their modest war loads on the British populace or targets-of-opportunity. Engineers estimated their new design to have an endurance window of some eighteen hours of flight time which was a very optimistic proposal.
For power the design team selected a pair of Anzani 9-cylinder air-cooled radial piston engines, each outputting 100 horsepower and driving multi-bladed propeller units in typical puller fashion.
Proposed armament was centered on 1 x 37mm Davis Gun cannon (detailed in the Small Arms section of this site) at the nose (also included here was a generator-fueled searchlight) fed by a stock of twenty projectiles. Supporting this was a trainable 7.7mm machine gun aft of the Davis Gun's position as well as a second 7.7mm machine gun also located aft of the top wing. All told, this provided the crew with formidable firepower at range.
Flight testing of the first of two P.B.31E prototypes was undertaken in the early-middle part of 1917 but its failings quickly shown through. The aircraft only managed a speed of 60 miles per hour (far short of the advertised 75mph limit) and its rate-of-climb proved poor as it took sixty minutes to reach 10,000 feet. On top of this was the propensity for the already unreliable Anzani series engines to overheat when pushed. All this worked against the P.B.31E seeing maturity in its prototype stage.
With such a showing, work was stopped on the second, still-incomplete prototype and the first prototype was scrapped as soon as July 23rd, 1917 - ending this first Supermarine venture in full.
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