Key point: The Montanas were designed to fight a different World War II than the one that happened.
In the early 1940s, the U.S. Navy still expected to need huge, first rate battleships to fight the best that Japan and Germany had to offer. The North Carolina, South Dakota, and Iowa class battleships all involved design compromises. The Montanas, the last battleships designed by the U.S. Navy (USN), would not.
Origins of the Design:
The interwar system of naval treaties allowed the United States to restart battleship construction in the late 1930s. The first designs (the North Carolina and South Dakota classes) complied with the restrictions of the treaties, which limited battleship size to 35,000 tons. An escalator clause kicked in after Japan failed to renew its treaty obligations, allowing the construction of the 45,000 Iowa class, which would use the extra displacement to carry slightly heavier guns, and more importantly to add five knots of speed.
The fast, slim Iowas class ships, however, diverged from the historical practice in battleship construction in the United States. Unlike their counterparts in other countries, U.S. battleship admirals preferred to sacrifice speed for firepower and protection. The USN never built any battlecruisers (although it intended to do so after World War I), and initially expected its replacement battleships to sail at 23 knots, four knots slower than any foreign contemporary. The USN bumped that to 28 when it realized foreign navies were building ships that could make closer to 30 knots. The Iowas could make 33 knots (at least on paper) because the USN wanted battleships that could escort its new fast carriers.
The Montanas didn’t drop all the way back to 23, but they did represent a step back to the precedent established by the North Carolina and South Dakota classes. They displaced 18,000 tons more than the Iowas, but spent that displacement on armor and main battery, rather than on speed. Crucially, the USN made the decision to build them too large to pass through the Panama Canal, which Japanese designers of the time had believed was a hard ceiling on the size of U.S. battleships.