That’s how it all started. Guys in planes with handguns.
The French eventually put a forward-mounted gun on the plane but had to install deflectors on the prop that would protect it from bullets. On the German side Fokker developed an interrupter gear to be mounted onto the Fokker Eindekker which prevented the mounted gun from discharging when the propeller was in the way. It wasn’t perfect, but better than the deflectors.
ETA: The story goes that Fokker himself went up to demonstrate the forward-mounted machine-gun with the interrupter gear, but once he got behind an Allied scouting plane, he didn’t have the heart to kill the crew. It didn’t take long, before other pilots gladly started shooting down enemy planes.
With biplanes, guns were sometimes mounted on the upper wing to evade the problem, though eventually the central powers developed their own interrupter gear mechanism.
Note that those flying contraptions were considered more valuable than pilots, and they were sent up without parachutes in order to given them incentive to return with the plane, or at least get it to the ground with less damage. As I flew WWI flying simulations, I noticed I had a while to think up some good last words while staring at the looming ground. Too bad no one would ever hear them.