“When Col. Samuel Lyman Marshall came home in 1945, he was one of millions of Americans who had served in the Second World War. Perhaps a third of them had seen combat, and Marshall, as the European theater’s deputy historian, had talked to an unprecedentedly large number of them. In a few months he began the little book that was to make him S. L. A. Marshall, a respected and highly influential military historian. In the 211 pages of Men Against Fire, Marshall made an astonishing assertion: In any given body of American infantry in combat, no more than one-fifth, and generally as few as 15 percent, had ever fired their weapons at an enemy, indeed ever fired their weapons at all.”
Source American Heritage magazine March 1989.
I wonder how prevalent this was in WW2 were troops tended to be more spread out than say the Napoleonic wars. Also there are some films which have cases like Band of Brothers and eternal zero wher the pilot avoided combat. Besides, the Spitfire pilot Wellum who wrote about combat fatigue felt massive remorse after killing an unsuspected snapper (ME-109 from behind). Killing people is not as easy as pixels.
Here is a link to more research.(which partly argues against Marshall.
.The Wookie 
is the one I took my solo Flights. I also got a “It’s dangerous to go solo, take a Wookie” shirt. Well they are the best copilots in the Universe. Moreover a copilot that can beat up Gators 
is useful when flying over Florida State 
.