1) Shinano was originally intended as a battleship. The English translation of "KOJINSHA No.6" states:
"The 74th Imperial Conference convened on 26 December 1938 approved the building of 83 naval vessels included in the new program. In addition to two battleships (SHINANO and No. 111), the Fourth Replenishment Program authorized the construction of a new fleet carrier...."
2) Shinano was then converted to an aircraft carrier. Many books contain claims like this:
"...Shinano, which was laid down as the third Yamato-class battleship in 1940 but was redesigned as an armoured carrier, the world's largest, after Midway. She was intended to carry few planes (up to fourty-seven) for her size but large stocks of fuel, munitions, and spares to replenish other carriers..."
as given in:
The Pacific Campaign: World War II: the Us-Japanese Naval War, 1941-1945 by Daan Van Der Vat
However although this seems to be a frequent claim, I cannot find a book that actually cites an original source for such statements.
Some interesting discussion here: http://propnturret.com/tully/viewtopic.php?f=2&t=1256 seems to support the argument she was intended for a replenishment role, but also lacks original sources.