During the second world war, the size of the U.S. military swelled significantly and became far larger than even during World War I. With technology advancing so significantly in the interwar years and the military growing more diverse, the need arose for a rank higher than four-star general. The U.S. needed an equivalent to the British and Russian Field Marshal.
But they declined. Instead, the five-star "General of the Army" and "Fleet Admiral" for the Navy were created by act of Congress.
The reason was ultimately that the name of U.S. Army Chief of Staff General George C. Marshall would have sounded ridiculous with this new rank. Still, the Americans were now the senior partner in the alliance against the Axis and its commanders were technically outranked by British Field Marshals.
Admiral Ernest King was a well-known stickler for things like uniform wear (he even introduced a new Navy uniform during the war), awarding fewer medals so that they meant something when awarded, and – especially – for rank. The longtime officer hated the idea of using British terms like "Admiral of the Fleet." He suggested other terms, like "Arch Admiral" and "Arch General." His suggestions were so odd, his contemporaries thought he was joking.
President Franklin Roosevelt thought "Chief General" and "Chief Admiral" would be good names for the new positions. The debate ended when Congress revived the five-star ranks with their new names in 1944.
Only one American officer ever held the title of "Field Marshal." Douglas MacArthur was appointed as Field Marshal of the Army of the Philippines in 1936 when the island nation achieved a semi-independent status. MacArthur was tasked to create an operational army for the fledgling country and wore a special uniform, complete with a Field Marshal's baton.