Captain America recieving his trademark circular shield from FDR in June, 1941. From Captain America #255 (March, 1981) (Roger Stern, writer; John Byrne, artist).
The shield’s history developed slowly. Originally, in Captain America Comics #1 (March 1941), Cap carried a heraldic-style shield made of steel. By issue #2, it had been replaced—without explanation—with the now-familiar circular design. In reality, the change had come about as a result of copyright issues; another comic book company had complained that Cap and his shield were a rip-off of a character called simply The Shield.
In 1981, it was revealed that the circular shield had been presented to Captain America by FDR in the Oval Office. Its creator was a metallurgist named Myron MacLain. While the original shield had simply been made of steel, MacLain’s was composed of an alloy of steel and the near-indestructable metal vibranium.
In a 2001 story, King T’Chaka of Wakanda meets Captain America in early 1941 and gives him the sample of vibranium from which the shield is made. A 2010 storyline presents a different version: Captain America and Nick Fury meet T’Chaka and his father during World War II, and—after the Red Skull crushes Cap’s triangular shield in a battle—T’Chaka loans Cap a circular vibranium shield that inspires Cap to replace the destroyed shield permanently with a circular one.