The AN-S-31 flight suit was developed for the US Army Air Corps. Previously they were wearing leather because cockpits were unheated but once they pressurized and temperature controlled cabins, a lighter uniform was needed. It also needed pockets, fire resistance, and other features. The flight suit was designed with these requirements in mind and hasn't needed to change much since.

Greetings & happy new year, flyers! Just a reminder that this sub is not the place for personal anecdotes, nor discussions of current flight wear. Let's keep the discussion historical (pre 1995) and on topic: "When and why did the modern "onesie" design come about?". Thanks!