Aviation came of age in WWI. Eleven years after the Wright brothers flew at Kitty Hawk, men in airplanes were shooting at each other over the Western Front, with pistols and rifles at first. Fire and maneuver are the basic elements of warfare, and nowhere did these elements fit better than in this new medium. Machine guns, both fixed and flexible, were in use by 1915. A major breakthrough was the invention, by German engineers, of a synchronizing device which made it possible for guns to be fired through the arcs of propellers.
By the end of the war, both sides employed multi engine aircraft to deliver bombs of up to 500lbs. in weight against strategic targets. An interesting development on the part of Germany was the construction of rigid airships; called Zeppelins, for the inventor, which were employed in night-bombing operations against targets in England and France. One notable handicap to effective air operation in WWI was the failure of either side to devise an accurate aiming system.
The Lewis gun, with or without its air-cooling jacket, was the standard observer's gun in the Allied air services. The revolving drum from which the gun was fed increased in capacity from 47 to 97 rounds in 1916. The Parabellum was used by German observers as was the Spandau. While the name Maxim gun was universal among the Allies for these designs - the Germans simply referred to them as LMG.08 or LMG.08/15 - which were the aviation models of the 1908 Maxim gun.
In Britain, the Maxim gun was manufactured by the Vickers Company and was known as a Vickers gun.
The American guns, Colt and Browning, were used during the war as was the Marlin, an improved version of the Colt.
A variety of other guns was used at one time or another during the war but the most important from the point of view of numbers used were the Lewis, Vickers, Parabellum, and Spandau.