An earlier Grossflugzeugkreuzer proposal, with two more 280mm breech loaders but two fewer 37mm anti-aircraft guns.

Note that both of these projects were actually proposed by the German government, but not built.

A Swedish coastal defense battleship proposed in 1945. Why it has 210mm breech loaders when the Swedes had 280mm ones at their disposal is beyond me, but I like those 57mm anti-air guns would be rather interesting in action.

A monitor to go with the aforementioned battleship.

Designed to essentially be a mobile shore battery, it would have relieved a lot of potential stress on the Swedish army.

This ship has a rather interesting story. Back when the Soviet Union was actually an ally to us, they asked Gibbs and Cox to design and build them a hybrid battleship/aircraft carrier. This was one of the proposals. Boasting more firepower than any battleship built besides the Yamato, and up to 40 aircraft, it was actually a highly efficient design displacing under 70,000 tons. The Russians still found it too expensive, however, and decided upon less ambitious proposals. By then, however, the invasion of Poland prevented any sort of agreement with the States.

Interestingly, it is designed with entirely American components, which suggests that if it did happen to be built it could easily be converted to American service (Like what happened with HMS Erin in the Royal Navy).

I'm skipping Tauchschiff Projekt 50, as the filesize is too great for 99chan's limit.

Grosskampschiffe 4541 was the culmination of an Imperial German effort to design a battlecruiser in place of L 20 e Alpha, as the Kaiser felt the L 20 was too slow for his needs. However, modifications to the L 20 design ultimately yielded a ship fast enough, putting the GK 4541 to an end. Of course, L 20 e Alpha was never built either, as the war ended less than a month later.

On to the real meat, the US Navy "Spring Styles" books.

We start with this peculiar design - a "torpedo battleship" - which sacrifices big gun armament for a crapload of submerged torpedo tubes. None were built, obviously.

Another interesting class of unbuilt ships, the "Battle Scout". If a battlecruiser is a giant heavy cruiser, presumably a battle scout is a giant light (scout) cruiser. This one has eight 14-inch guns, but there were other proposals such as four 16-inch ones.

The infamous Tillman IV-2 maximum battleship, demanded by Congress to investigate the largest war vessel the United States could build without having to redo the entire Panama Canal to incorporate it.

There is also a similar design with 20-inch guns proposed in 1934, but since nobody knows what happened to the second Spring Styles book, many of its aspects remain a mystery.

A miniature battleship with four sixteen-inch guns and a top speed of 29 knots.

Sounds a lot like the panzerschiffe of the German navy.

A self-propelled dry dock.

Auxiliary vessels can be interesting too!

A colossal "cruiser submarine" that ultimately was never built.

Two even more ambitious proposals existed, but information on them is scarce at the moment.

A "flight deck cruiser" combining light cruiser sized armament with 36 scout-bombers.

I guess KanColle should rename their CAVs to CFs, huh.

A cruiser-killer requirement that ultimately led to the Alaska class also yielded this unused proposal based on an Iowa hull.

Possibly the ultimate battleship proposed in the United States navy, this fast Montana also boasted an insanely high shaft horsepower only obtainable through a return to turbo-electric drive. Ultimately the proposal was deemed so ambitious it was felt that it should make up the Montana's never-were successor, and development was indefinitely halted.

Apparently Des Moines and Worcester would've had a nice little sister in the form of the abortive CL-154.

The power of fully automatic loading was phenomenal when first developed. In particular I have special respect for the 8"/55RFs used on Des Moines.