One of the key developments in World War I aerial combat was the synchronizing gear that allowed machine guns to be fired through the prop in front of the pilot without shooting said prop.
The gun synchronizer, an interrupter that linked the passage of an airplane’s prop with the firing of a machine gun set directly behind it, was pioneered in the years directly before the Great War but it was the German Eindecker, a revolutionary single-wing monoplane fighter introduced in 1915 that flew with the first (kinda) reliable synchronization gear, and soon cleared the sky of British and French airplanes.
Before then, pilots and aircrew were limited to firing rifles and handguns at passing aviators and mounting machine guns overhead on the wings, none of which proved very good when it came to aiming.
As the war wound on, the use of synchronizing gear became widespread on both sides and remained standard in the air forces of the world to some extent into the Second World War.
With all that being said, The Slow Mo Guys take a great look at a low-speed prop set up with a vismodded “Vickers” machine gun to get the idea of just how such a gear works– and what happens when it does not.