United States Navy were planned as successors to the Iowa class, being slower but larger, better armored, and having superior firepower. Five were approved for construction during World War II but changes in wartime building priorities resulted in their cancellation in favor of the Essex-class aircraft carriers before any Montana-class keels were laid.With an intended armament of twelve 16-inch (410 mm) guns and a greater anti-aircraft capability than the preceding Iowa class, the Montanas would have been the largest and the most heavily-armed battleships put to sea by the United States. They would have been the only US Navy battleship class to have come close to rivaling the Empire of Japan's Yamato-class battleships in terms of armor, weaponry, and displacement.Preliminary design work for the Montanas began before the US entry into World War II. The first two vessels were approved by Congress in 1939 following the passage of the Second Vinson Act. The Japanese attack on Pearl Harbor delayed construction of the Montana class.