By the time the first prototype had been completed the RAF 4D was not sufficiently developed, so it was fitted with the alternative Hispano Suiza engine. On 30 November 1917 the first prototype, flown by Captain Gordon Bell, made its maiden flight from Royal Flying Corps Station Joyce Green, Kent. In January 1918 the first prototype was dispatched to RAF Martlesham Heath, Suffolk, for the official trials of the type. Reportedly, the F.B.27 quickly made a positive impression: it was able to take off with a greater payload than the Handley Page O/400 despite having about half the effective engine power. The engines proved to be unreliable during these trials, leading to the aircraft's return to Joyce Green on 12 April 1918.