AskMen.com has an article which you can view by clicking here, titled GMT & World-Time Watches and they say "The archetype is the Rolex GMT-Master II, which was originally engineered for Pan-Am Pilots in the 1950s and 1960s." The article points out the significance of being able to keep track of time in two different places simultaneously.
Since the article did not show the Pan Am GMT Master, I thought I would share a few shots with you that Marlon took of his utra-rare white dial GMT Master. On Jake's Rolex Watch Blog we refer to this rare combination as a Vanilla Pepsi. As the folklore goes, Rolex developed the GMT Master for the Pan Am pilots with the standard black dial, and for the executives they developed a rare white (silvered dial) as seen in Marlon's photo below.