The Mail Gun
Ernest K. Gann, in his autobiography "Fate is the Hunter", recalls bitching about having to carry The Mail Gun when he was an airline pilot for maybe United, I think it was, back in the late thirties. He wrote about how relieved he and his fellow airline pilots were, to hear that the rules had been relaxed slightly at the insistence of their employer, so they didn't actually have to wear the gunbelt for the whole flight.
As far as I know, airline pilots were *required*, no discretion allowed, to carry pistols when operating an airplane carrying the U.S. Mail as late as 1967.
Edit: Changed "novel" to "autobiography". Sorry, it's a better story than most novels.