Design B-65 was a class of cruisers planned by the Imperial Japanese Navy (IJN) before and during World War II.

The Decisive Battle concept was the IJN's primary strategy for defeating the navies of the major Western powers, such as Great Britain or the United States, in the event of war.

The strategy consisted of four main components: a decisive surface fleet engagement determined by big guns, attrition tactics against a numerically superior enemy, a reliance on quality over quantity in naval weaponry, and the employment of nighttime torpedo attacks.

After cracking the outer defense ring, IJN cruisers and destroyers were to launch torpedo attacks on the enemy's battleships.

As envisioned by the IJN in 1936, these ships, the B-65 cruisers, would mount 310 mm (12.2 in) guns, carry armor designed to withstand hits from 203mm (8 in) shells, and be capable of speeds up to 40 knots.

[5] Preliminary plans for the new class were finished by September 1940, and they were far enough along that tests of the lower protection—the armor intended to counter torpedoes and any shells that fell short but still hit the ship (albeit underwater)—and main armament were conducted from 1940 to 1941.

[1] These plans called for ships that bore a striking resemblance to the Yamato-class battleships, with the same "clipper bow, flush-deck construction, and a generally similar superstructure", albeit at a reduced size.

[13][16][A 2] As war with the United States loomed in 1941, the navy found that it had a more pressing need for aircraft carriers and auxiliary vessels.