Secret British or American 18-inch battleships

If Britain or the United States secretly built a huge 18-inch-gun battleship before World War II, what would it look like?

Britain would likely enlarge a King George V-era design for nine 18-inch guns, while the United States would scale an Iowa or advance toward a Montana-style ship.

In this counterfactual, Britain could enlarge a King George V or related Lion design for nine 18-inch guns, assuming a suitable gun had been developed quietly. A larger British study with twelve 16-inch guns provides the suggested starting point for that substitution. American options include scaling up the Iowa or bringing a Montana-style ship forward into its place in the building programme, while continuing faster Iowas toward a battlecruiser role. One proposed British programme pairs four King George Vs with four 18-inch ships, revising an earlier suggestion of only two heavier units. Their size would strain industrial capacity; other proposed characteristics include a speed of 28 knots and armour against heavy shells. The hulls themselves would be difficult to conceal, but their true gun calibre could remain undisclosed until launch or fitting-out. Rival governments would therefore see large battleships under construction before learning that they carried 18-inch guns.