Between the last battleship class constructed just after World War I, the Marylands, and the next, the North Carolinas, was a gap of 17 years. These seventeen years guaranteed that the new battleship design would be vastly different from the older ships.  
       The decisive factor in the construction of the North Carolina  class certainly was the Washington Naval Arms Limitation Treaty of 1922, and the London Naval Treaty of 1936, replacing and adding to the earlier treaty.  
       The major problem in these ships' design was Japan. That country had threatened to abandon the treaties it had signed in 1934 (and would do so, in 1937), which essentially left the remaining signatories in a problematical situation. If they continued to adhere to the treaties, they might find themselves opposed by battleships much more capable than their own. If they didn't adhere to the treaties, a new arms race might follow, which no signatory desired.  
       Thus, a clause was inserted according to which the signatories were allowed to go over the treaty limitation of 35,000 tons, 14" guns, if a signatory cancelled or broke the treaty.  
       The North Carolina-class design started in 1934. Though the problems one could expect from such strict limitations as those of the Washington Treaty were encountered and solved, the major issue of the main gun caliber was not decided until the last minute. 406mm were prefered by President Franklin D. Roosevelt, among others, but 14" was the treaty standard and using it would be a sign to the other signatories that the U.S. desired adherence to the treaty. This problem led to a simple solution: if Japan adhered to the 14" limit, so would the U.S. -- if not, 16" guns would be used. Thus the barbettes were constructed in such form that either of two turrets, a quadruple 14" or a triple 16", could be installed.  
       As it turned out, Japan did not desire to keep anywhere near the treaty limits, and in 1937, shortly before the keels for the two ships were laid, the 16" variant was ordered. It was too late, however, to also change the armor outfit, which remained set to protect the ship against 356mm shellfire, contrary to the old rule-of-thumb maxim that a battleship should be armored against guns of its own caliber.  
       As the first completed battleship design in 17 years, the North Carolinas suffered some serious vibration problems, sometimes effectively limiting their speed to less than their maximum. They did introduce the torpedo protection system that became the basis of all later battleship designs.  
       Both battleships saw extensive service during the war. North Carolina herself was first to go to the Pacific in June, 1942, after the Battle of Midway, having spent most of her prior career off the Eastern seaboard. She escorted carriers until torpedoed by a Japanese submarine, returned to the Solomons after her repairs, and supported the carriers in all major campaigns of the Pacific War. She remains a floating museum on the Cape Fear River near Wilmington, North Carolina.  
       USS Washington served a brief stint with the British Home Fleet guarding against a breakout of Tirpitz, then was sent to the Pacific in November of 1942. She fought successfully in the Second Naval Battle of Guadalcanal, sinking the Imperial Japanese battleship Kirishima with 16" fire, then followed much the same career as North Carolina, being scrapped in 1961. 

Ships in class: 
BB-55 North Carolina 
BB-56 Washington