Aviation was in its infancy, and at first planes were used for reconnaissance, because they hadn't yet figured out how to mount guns to them. Pilots on opposing sides would wave to each other, feeling a certain congeniality.
Eventually things turned more bitter, and pilots started carrying pistols or rifles to snipe at opposing pilots. I've even read of a brick being thrown.
Later, machine guns were mounted in front of the pilot, but they hadn't figured out how not to shoot off the plane's own propeller. An early solution was to reinforce the propeller with metal plates designed to ricochet bullets away, and just shoot anyway. At a demonstration of this technology, ricochets killed a bystander.
Later still, they figured out how to synchronize the firing of the guns with the turning of the propeller, so as to fire between the blades.
An early Nieuport warplane was infamous for its wings falling off in steep dives. To test just how steeply the plane was capable of diving, one ace took it up and dove at increasingly steep angle, until the wings began to disintegrate. This knowledge was helpful to other pilots.
The USA's most capable ace, Eddie Rickenbacker, was a former race car driver, who theorized that given their familiarity with engines and fast speeds, race car drivers would make excellent fighter pilots. He also Americanized his name from Richenbacher so it would sound less German. He had 26 confirmed kills, the most of any American pilot.
The Red Baron, Manfred Von Richthofen, had 80 confirmed kills, the most of any pilot in the war. His younger brother Lothar amassed 40.