The Vickers F.B.5 (Fighting Biplane 5) (known as the "Gunbus") was a British two-seat pusher military biplane of the First World War. Armed with a single .303 in (7.7 mm) Lewis gun operated by the observer in the front of the nacelle, it was the first aircraft purpose-built for air-to-air combat to see service, making it the world's first operational fighter aircraft. Design and development Vickers began experimenting with the concept of an armed warplane designed to destroy other aircraft in 1912. The first resulting aircraft was the "Destroyer" (later designated Vickers E.F.B.1) which was shown at the Olympia Aero Show in February 1913, but crashed on its maiden flight. This aircraft was of the "Farman" pusher layout, to avoid the problem of firing through a tractor propeller, and was armed with a single belt-fed Vickers gun. Vickers continued to pursue the development of armed pusher biplanes, and their Chief Designer Archie Low drew up a new design, the Vickers Type 18, or Vickers E.F.B.2. This was a two-bay biplane powered by a single 80 hp (60 kW) Gnome Monosoupape nine-cylinder rotary engine; the aircraft had a steel tube structure, with fabric-covered wings and tail, and a duralumin-covered nacelle with large celluloid windows in the sides. The unequal-span wings were unstaggered, with lateral control by wing warping, while the aircraft had a large semi-circular tailplane. Armament remained a single Vickers gun mounted in the nose of the nacelle, with limited movement possible, and a very poor view for the gunner. The E.F.B.2 made its first flight at Brooklands on 26 November 1913. It was soon followed by the E.F.B.3, powered by a similar engine, but using ailerons instead of wing warping, and with equal-span wings, while the nacelle omitted the large windows fitted to the E.F.B.2. The belt-fed machine gun proved problematic, and the weapon was changed to the lighter, handier, drum-fed .303 in (7.7 mm) Lewis gun. The F.B.5 first flew on 17 July 1914. It was powered by a single 100 hp (75 kW) Gnome Monosoupape nine-cylinder rotary engine driving a two-bladed propeller, and was of simple, clean, and conventional design compared with its predecessors. In total, 224 F.B.5s were produced, 119 in Britain by Vickers, 99 in France and six in Denmark. F.B.6 The Vickers F.B.6 was a development of the F.B.5 with an increased span on the upper wing. Only one was built. F.B.9 A further