Like a lot of men's clothing the Tee Shirt started as an undergarment. Though exactly who originated it is up for discussion. What we do know is a couple things happened simultaneously.
The union suit, a one piece undergarment, was separated into two pieces. A long under-trouser and a separate top. Some have suggested that this separate top was modified into what we’d call a Tee Shirt.
Sometime between the Spanish-American War and WWI the US Navy began issuing short sleeve undershirts as part of the standard uniform. If it was a fresh idea or co-opted from the British who had a similar flannel undergarment is up for debate.
From the Southern California point of view, the "real" T-shirt was born in LA. Articles from the Los Angeles Times' archives point to the year 1932, when legendary USC football coach Howard Jones and athletic director Bill Hunter asked Jockey International Inc. to create a sweat-absorbent undershirt that would keep shoulder pads from chafing players' skin. The shirts, stenciled with "Property of USC" to discourage theft, became wildly popular with students.
No mater who holds the trophy for coming up with the idea, it was photos of our greatest generation fighting during WWII that the Tee took it's place on the pedestal of cool. Not long after, Hollywood reached a zenith of T-shirt popularity. In 1951 brought "A Streetcar Named Desire," with Marlon Brando bellowing, "Hey, Stella!" in a ripped thin T-shirt meant to represent the thin layer that held back Stanley’s inner animal. Brando wore a Tee under leather in 1953's "The Wild One." Then James Dean created a classic image in white tee and blue denim in 1955's "Rebel Without a Cause.”
Shortly after, the style of dress was becoming more casual and sporty and undergarments were becoming outer garments solidifying the Tee shirt as a clothing essential.