Sopwith Gunbus The Sopwith Gunbus was a British fighter aircraft of the First World War . It was a single - engined pusher biplane based on a floatplane built by Sopwith before the war for Greece . Small numbers were built and used by the British Royal Naval Air Service , mainly as a trainer . Development and design . In 1913 , the Sopwith Aviation Company received an order for six two - seat floatplane from the Greek Government for the Greek Naval Air Service , which was in the process of being set up on the basis of advice from Rear Admiral Mark Kerr , the head of the British Naval Mission to Greece . Sopwith ' s design , known as the " Greek Seaplane " or " Pusher Seaplane " was a single - engined pusher biplane powered by a single 100 hp ( 75 kW ) Anzani radial engine , with four - bay wings . It was fitted with dual controls for use as a trainer . Operational history . The first of the Greek Pusher Seaplanes flew in February 1914 , successfully passing trials in March , with first deliveries in May and all six delivered by the outbreak of the First World War . Two more identical trainers were purchased by the Royal Naval Air Service , again for use as trainers , these being delivered in May . While the Greek machines performed well , despite the limited facilities available at their base at Eleusina , with at first no workshops or hangars available , the two British aircraft were less successful , with their engines proving unreliable , and were withdrawn by February 1915 . In March 1914 , the Greeks placed an order for six more pusher seaplanes , the Sopwith S PG N , which were similar to their previous aircraft , but rather than being dual control trainers , were to be armed with a machine gun in the nose , and powered by a Gnome Monosoupape rotary engine . At least five of these aircraft were taken over by the Royal Navy on the outbreak of war , but again proved unsuccessful in British Service , with at least two being modified as landplanes . They remained in service until July 1915 . The Royal Navy ordered six modified landplane based on the S PG N in July 1914 , the Sopwith Gunbus or Admiralty Type 806 , to be powered by 110 hp ( 82 kW ) Sunbeam water - cooled V8 engines and armed with a machine gun . The first one flew on 6 October 1914 , and was found to be underpowered , so was fitted with a 150 hp ( 112 kW ) Sunbeam . A further 30 aircraft were ordered from Robey & Co . Ltd . of Lincoln in early 1915 , these being fitted with a modified nacelle , with the pilot sitting in the forward cockpit rather than the gunner , and fitted for bombing . Only 17 of these aircraft were completed , with the remaining 13 delivered as spare parts . The Sunbeam powered Gunbuses saw limited operational use , with one aircraft being on the strength of the RNAS squadron at Dunkirk led by Commander Charles Samson in February 1915 , with Samson commenting that the Sopwith required " a lot of work on it to make it safe to fly " . The Gunbuses were mainly used as trainers , being used by the RNAS at Hendon , and remaining in service until the winter of 1915 - 16 . The name " Gunbus " came from the Royal Flying Corps ( RFC ) pilot ' s slang term for an aeroplane - a " bus " - and was also used in the name of the Vickers Gunbus .