Are Commercial Airplanes vulnerable to gun fire?

BTW, that door gunner was initially cast as Gunnery Sgt. Hartman.

I’m 90% sure this is a Far Side cartoon, either that or it’s a birdhunter who shot down a UFO and the caption was “What really happened at Roswell”.

Bear in mind WWI biplanes were also really slow - a maximum top speed of about 110-140km/h or so. Hitting a moving target at any speed is difficult, but there’s a huge difference between one doing 100km/h and one doing 800km/h+