While ardently constructing impressive super-battleships, Japan reluctantly realized that heavily armored aircraft carriers were a better use of shipbuilding material. In a deal with powerful Navy "battleship admirals", a Shokaku-class CV currently under construction was made into battlecruiser Aratama and material was consolidated and set aside for super-battleship Yokozuna. In turn, the proposed Yamato-class battleships Musashi and Shinano would be converted into Japan's largest and best-protected aircraft carriers.

The Navy ordered the two partially-constructed battleships converted to CV's while on the slipways. As originally designed they were not intended to have an air complement of their own. Their shops and stores were to replenish planes from other, more vulnerable carriers, and their armored decks would provide more landing spaces for displaced aircraft and some shelter from bombs (oh, what these carriers could have done for the IJN at Midway!). A revised design added a small complement of the latest Aichi D4Y8 Susei-Kai attack bombers (Allied code-name "Judith") and Kawanishi A7K Jinpu-Kai fighters (Allied code-name "Pancho") to give an offensive punch. As Shinano was not nearly as far along in construction as Musashi, she was re-configured for substantially more aircraft, giving her as many aircraft as the Taiho/Ikoma design. Considering that the Taiho carried the same number of aircraft on roughly half the tonnage, and that Yonaga had carried twice the aircraft on similar tonnage, Musashi and Shinano were rather inefficient aircraft platforms. Nonetheless, their pilots were some of the last of the highly-trained aircrews before the "Emergency Pilot Program" lowered the standards, and they were eager to be assigned to the newest and largest CV's in the Imperial Navy (indeed the whole world).

Musashi and Shinano formed Carrier Division 6 and first served the Fleet during the closing phases of the Solomons campaign in mid-1943. The Division sortied from Truk, in the Carolines, directly south to disrupt the Allied invasion of New Georgia. No American carriers were involved, but off the Tauu islands a lucky B-17 "walked" a string of 500-lb bombs diagonally across the Musashi's armored flightdeck. Bomb splinters did some minor damage topside, but the deck held, and flight operations were not seriously impaired. Hopes rose for a favorable confrontation with the growing American carrier fleet in upcoming operations.

Buoyed by their success in fending off Allied air attacks, CarDiv6's role in the upcoming Marianas campaign changed. Originally designed to stay in the rear of the fleet as support ships, the heavily armored pair were put in the van to absorb the initial attacks so that the less-protected carriers had a better chance of surviving. Car. Div. 6's aircraft found and sank the American angled-deck carrier Bon Homme Richard off Saipan, but the two Japanese carriers suffered some damage from the American retribution attack at dusk and fell behind as the rest of the Japanese fleet fled at high speed. An unexpected night attack from radar-equipped Avengers slowed them down to barely a crawl, and when dawn broke both Japanese giants succumbed to massive air attacks from the new Douglas "Decimator" and Fairchild "Flounder" attack planes, requiring over thirty torpedoes (including the Flounders' "Highball"s) and 45 bombs to sink.