The original Final Fantasy, released in Japan on the Famicom in 1987, is not part of the JRPG subgenre, which didn't even exist yet. In fact, it's based more directly on D&D/AD&D than nearly all Western-made CRPGs, evident not only from Amano's concept art but also from the classes (which are effectively the fighter, monk, thief, cleric, magic-user, and multi-classed fighter/cleric/mage), the spell system (a Vancian magic system only slightly modified from D&D/AD&D), the actual spells available (e.g. 3rd level black mage spells include fireball FIR2 and lightning bolt LIT2 that affect all enemies, and 4th level black mage spells include ice storm ICE2), monsters (to such an extent that trolls are green with regenerative ability and weak against fire, while D&D-original beholders, mind flayers, sahuagin, otyughs, and the Type V Demon Marilith are all present), and just about every other aspect of the game.
However, the original Final Fantasy was not released in the United States until 1990, and so the following year Squaresoft brought to the SNES a "Final Fantasy II" that was actually the fourth game in the series and very much in the JRPG subgenre, with a narrative focus on pregenerated characters, as would be the case for all later FFs.