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 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.
Vickers began experimenting with the concept of an armed warplane designed to destroy other aircraft in 1912. The first resulting aircraft was the Type 18 "Destroyer" (Vickers E.F.B.1) which had been demonstrated in 1913. 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 Maxim machine gun. The belt feed proved problematic for a flexible machine gun, and the weapon installed was changed to the lighter, handier, drum-fed Lewis gun. The E.F.B.1 was the first in a line of Vickers' "Experimental Fighting Biplanes", of which the F.B.5 was the most famous - and the first to be built in quantity.
The F.B.5 first flew 17 July 1914. It was powered by a single 100-hp Gnome Monosoupape 9-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 6 in Denmark.
The F.B.5 began to be seen on the Western Front when the first examples reached No.2 Squadron RFC on 5 February 1915. The type served in ones and twos with several other units before No. 11 Squadron RFC became the world's first fighter squadron when, fully equipped with the F.B.5, it deployed to Villers-Bretonneux, France on 25 July 1915. Second Lieutenant G.S.M. Insall of 11 Squadron won the Victoria Cross for an action on 7 November 1915 in which he destroyed a German aircraft while flying a Gunbus. No. 18 Squadron RFC, which deployed to France in November 1915, also operated the F.B.5 exclusively.
The F.B.5's performance proved to be inadequate for its intended role; although its forward firing machine gun was a great advantage, the fighter did not have the speed or rate of climb to pursue its quarry. By the end of 1915 it was outclassed by the Fokker Eindecker. Some examples of the improved Vickers F.B.9 were sent to France, pending sufficient supplies of the Royal Aircraft Factory F.E.2b, but the active career of the "Gunbus" was soon over. The remaining examples were mostly used as trainers.
Despite its moderate effectiveness, the Vickers F.B.5 did have a lasting legacy as German pilots continued to refer to British pusher aircraft as "Vickers-types". Many victories over D.H.2 or F.E.2b pushers were reported as destruction of a "Vickers".