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Feb 1, 2011
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According to the Japanese Wiki:
There was a warship design which was proposed in 1939 for the Maru 4, Naval Armaments Supplement Program. This cruiser was to be based on Tone with same armament and arrangement (4 twin 20cm gun turrets all forward) but slightly larger displacement of 13.000tons standard. 2 ships to be built but the design never progressed beyond the proposal.
I've been intrigued by this design and decided to make a drawing of it which I now present here as well.

Because of so little information available, I had rather free hand to design this never were warship. The changes compared to the original Tone class heavy cruisers as follows: - Slightly different main armament arrangement: 3 facing forward, 1 aft resulting one shell handling room to be closer to the hull's centre. This should slightly help the ship's stability and weight distribution. - 10cm Type 98 DP-AA Guns which are designed at the time these warships proposed and are vastly superior to the 12,7cm Type 89 DP-AA Guns. - 4 quadruple torpedo launchers instead of the original triple, providing the same torpedo firepower as the later Ibuki class heavy cruisers. - More numerous and differently arranged triple 25mm Type 96 AA guns to provide heavier AA support - Slightly different superstructure and platforms with 4 instead of 3 large reflectors. - Seaplane hanger very similar to that used on the Oyodo class light cruiser and which able to carry 3 seaplanes of various types (Mitsubishi F1M Pete, Aichi E13A Jake or Kawanishi E15K Shiun/Norm) - 3 catapults to launch the seaplanes, 2 of the regular 19,4m long type and 1 45m long type for the E15K Shiun. - Total seaplane capacity: 3 on the catapults, 3 in the hanger and possible 1 more on the deck totalling to 6-7 seaplanes. Alternatively without the large 45m catapult the seaplane capacity could rise to: 2 on the catapults, 3 in the hanger and 3-4 on the deck totalling 7-9 aircrafts - Possible slightly stronger engines of approximately 160.000shp - Possible thicker armour of 127mm Machinery and 150-160mm Magazine belt with 75mm deck
Now the drawings:
As built or soon after commissioned:

A very likely 1943/44 refit for the Tone-Kai design similar to what happened to the IJN Oyodo:Removal of the 45m large catapult, using it's space to carry E13A Jake Floatplanes, more 25mm triple and twin AA guns and the green-light green camouflage pattern used on many IJN warships.

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