In the United States Navy, Admiral of the Navy is traditionally considered the equivalent of a six-star Admiral. The rank has only been held by one person in history: George Dewey. In recognition of his victory at Manila Bay in 1898, Congress authorized a single officer to hold the rank of Admiral, and promoted Dewey to this rank in March 1899. By a Congressional Act of 24 March 1903, Dewey's rank was established as Admiral of the Navy, effective retroactive to March 1899. It was specified that this rank was senior to the four-star rank of Admiral and was equal to Admiral of the Fleet in the British Royal Navy. The rank lapsed with the death of Admiral Dewey on 16 January, 1917.
The rank Admiral of the Navy gained its equivalence as a six star rank during the Second World War, with the establishment of the rank of five-star Fleet Admiral.
During the preparations for the invasion of Japan, a proposal was raised by the Navy Department to appoint Chester Nimitz to the rank of Admiral of the Navy and grant him an insignia as a six-star admiral. Proposals for the new six star rank included changing the title to Flag Admiral. The proposal, however, was dropped and the United States Navy has never officially appointed anyone to the rank of six star admiral.
In the United States Army, the equivalent of Admiral of the Navy is known as General of the Armies.