Who invented the T-shirt? During World War I, US soldiers, sweating in their woolen uniforms, noticed the light cotton undershirts worn by European soldiers. Since they were much more comfortable, they quickly became popular among the Americans, and because of their design, they were named T-shirts. During World War II, the T-shirt became standard issue underwear in both the US Army and Navy. After WWII, the T-shirt started appearing without a shirt covering it.