In World War I, aircraft were initially intended for aerial reconnaissance, however some pilots began to carry rifles in case they spotted enemy planes.
The Airco DH.2 pusher plane had its gun in the front while the engine was in the back, some experimented with mountings on the (side) wing or on the biplane's upper wing (above the cockpit), until by 1916 most fighter aircraft mounted their guns in the forward fuselage using a synchronization gear so that the bullets did not strike the propeller.
In World War II, fighter aircraft carried machine guns and cannons mounted in the wings, engine cowlings, nose, or between the banks of the engine, firing through the propeller spinner.
While missiles have been the primary armament since the early 1960s, the Vietnam War showed that guns still had a role to play and most fighters built since then are fitted with cannons (typically between 20 and 30 mm in caliber) as an adjunct to missiles.
Modern European fighter aircraft are usually equipped with the revolver cannon, whereas the United States and to some extent Russia generally favor the Gatling gun.