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An aircraft constructed with a tractor configuration has the engine mounted with the propeller in front of it so that the aircraft is "pulled" through the air, as opposed to the pusher configuration, in which the propeller is behind the engine and "pushes" the aircraft forward.
In the early years of powered aviation both tractor and pusher designs were common. However, by the midpoint of the First World War, interest in pushers declined and the tractor configuration dominated. Today, propeller-driven aircraft are assumed to be tractors unless it is stated otherwise.
Early usage of term
As early as 1910, in the early years of flying, a distinction was made between a propeller ("pushes the machine", akin to a ship's propeller) and a tractor-[air]screw ("pulls the machine through the air") 
World War I military aviation
From a military perspective, the problem with single-engine tractor aircraft was that it was not 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 synchronization gear, more properly known as a gun synchronizer, developed by Anthony Fokker and fitted to the Fokker E.I monoplane in 1915. The first British "tractor" to be specifically designed 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 hub or spinner (the nose of the aircraft) — first used in production military aircraft with the World War I French SPAD S.XII — or mounting guns in the wings. The latter solution was generally used from the early 1930s until the beginning of the jet age.
- "Propellers and tractor-screws" Correspondence Flight 23 April 1910.