One of the most important advances in the early days of aerial combat took place during WWI. During the early part of the war, the only planes that could mount effective gunnery were the two-seaters, where the observer had a swivel-mounted machine gun, which he used while the pilot flew. In single seaters, the problem was that the pilot either had overhead or wing guns, which were difficult to load and aim, and often jammed, or a front-mounted gun, with which he would shoot off his own propeller in short order.
After a brief French experiment with armored propellers, German engineer Heinrich Lübbe and designer Anthony Fokker developed a solution: an "interrupter gear" which caused the machine gun not to fire when one of the propeller's blades was in front of the muzzle. The German air force used this new technology to hold control in the air over the trenches in the year-long "Fokker Scourge," until the allies gained equivalent technology in 1916.
German pilot and Ace Max Immelmann shot down 17 Allied aircraft using this new tool before he was killed in action in June of 1916.