The Citation II (Model 550) is a larger stretched development of the Citation I (Model 500). In addition to more seats, the Citation II has the more powerful JT15D4 engines, faster speeds and longer range.
Cessna later announced they would be improving the aircraft, and the upgraded Citation S/II (Model S550) was introduced. The aircraft utilized an improved version of the engine, JT15D4B, while the rest of the improvements were aerodynamic in nature. The wing was replaced with one using a supercritical airfoil, which had been developed for the Citation III.
However, due to market demands, the Citation II was later returned to production. Eventually the Citation II and Citation S/II designs led to the development and introduction of the Citation Bravo.