The undershirt, in the form of the white crew neck tee specifically, came about in the early part of the 20th century, when the US Navy issued their marines with white crew neck t-shirts. Their purpose was to absorb sweat primarily, but also they were used as a second layer that the marines could get dirty, to preserve the cleanliness of their uniforms.
After World War II veterans continued to wear their uniform pants and undershirts casually and a trend began to emerge. The white crew tee was born into everyday society and it wasn't just being used as an undershirt anymore!
Sports teams and other institutions were quick to adopt the white crew tee in the same way as the Navy did, and once it began to make it's appearance on big screen Hollywood movies an epidemic had started. Marlon Brando and James Dean were pioneers of this look which spread like wildfire with the youth of America.
Today the white crew tee is regarded as one of the most iconic pieces in fashion history and still remains an everyday essential for the average guy on the street as well as being a fashion item in it's own right.