When Captain America punched Adolf Hitler on the cover of Captain America Comics #1, the hero was carrying a triangular-shaped shield. By the second issue of Captain America Comics, the triangular shield had been replaced by the current, iconic disc-shaped shield.
The shield was changed due to complaints made to Captain America’s publisher, Timely Comics (now Marvel Comics), by MLJ Magazines (now Archie Comics), the publisher of another patriotic superhero character, The Shield. The Shield debuted in Pep Comics #1 (January 1940) over a year before Captain America debuted in Captain America Comics #1 (March 1941).
MLJ Magazines felt that Captain America’s shield was too close in appearance to the chestplate of The Shield. To avoid any potential legal challenges, Timely changed the shield design for Captain America in time for Captain America Comics #2.