From Chuck.

Tom Wolfe's book The Right Stuff profiles Chuck Yeager, the first pilot to break the sound barrier(and live ). Chuck is a country boy from the hills of West Virginia.

Wiki: Yeager's drawling manner of speech during flight (which Tom Wolfe refers to as "poker-hollow") was emulated by his fellow test pilots after he broke the sound barrier. This permeated into the general pool of military pilots, who became the core of the U.S. airline pilot roster, and so was the basis for the characteristic tone used by American airline pilots to this day