X-Plane 9.70+ Vickers F.B.5 Gunbus. The Vickers FB5 (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. By the end of 1915, it was outclassed by the Fokker Eindecker. A flying replica of the F.B.5 Gunbus was completed in 1966 and flew until late 1968 and now resides as an exhibit at the Royal Air Force Museum at Hendon near London. By Philippe Gastebois.