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An aircraft constructed with a tractor configuration has the engine mounted with the propeller facing forwards such that the aircraft is "pulled" through the air, as opposed to the pusher configuration in which the propeller faces backwards and the aircraft is "pushed" through the air.
In the early years of powered aviation both tractor and pusher designs were common. However, by the mid-point of the First World War, interests in pushers declined and the tractor configuration dominated such that today all propeller-driven aircraft are assumed to be tractors unless stated otherwise.
From a military perspective, the problem with single-engine tractor aircraft was that it was not originally possible to fire a gun through the propeller arc without striking the propeller blades with bullets. Early solutions included mounting guns (rifles or machine guns) to fire around the propeller arc, either at an angle to the side — which made aiming difficult — or on the top wing of a biplane so that the bullets passed over the top of the propeller.
The first system to fire through the propeller was developed by French engineer Eugene Gilbert for Morane-Saulnier and involved fitting metal "deflector wedges" to the propeller blades of a Morane-Saulnier L monoplane. It was employed with immediate success by French aviator Roland Garros and was also used on at least one Sopwith Tabloid of the Royal Naval Air Service.
The final solution was the interrupter gear, also known as "synchronization gear", developed by Fokker and fitted to the Fokker E.I monoplane in 1915. The first British "tractor" to be fitted with synchronization gear was the Sopwith 1½ Strutter which did not enter service until early 1916.
Other solutions to avoiding the propeller arc include passing the gun's barrel through the propeller's spinner (the nose of the aircraft) or mounting guns in the wings. The latter solution was generally used from the early 1930s until the beginning of the jet age.
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