Prior to 1920, USN warships were catergorized by one letter and then a number, such as B-1 (Battleship), D-1 (Destroyer), S-1 (Submarine), etc. But by 1920, the USN was becoming swamped with new types of vessels...aircraft carriers, submarines, patrol boats, support vessels, old battleships, new Dreadnaught battleships, Armored Cruisers, Protected Cruisers, etc.
In 1920, a second letter had to be added; such as AC for Armored Cruiser, DE for Destroyer Escort, and then the hull number added to that. For the sake of consistency, the first letter "B" for battleship meant BATTLESHIP, but the second "B" was simply for uniformity. This also applied to the Destroyers (DD).