If you count the A-12/SR-71 as first generation stealth (and it was intentionally designed into the aircraft), then the F-117A would represent 2nd generation, the B-2 would represent the 3rd generation, and the F-22 the 4th.

According to what I've read and heard, Lockheed was under pressure to lower the Oxcart's RCS after it was learned that the Soviets were able to track a U-2 from very long distances. The A-11 configuration looked pretty much like the final product, only they built in more composites and other radar absorbing tricks to produce the A-12. The YF-12A and SR-71A were derivatives of the basic A-12 design, deleting the overflight camera and adding a 2nd crewmember (the A-12 was a single seater). They didn't compromise speed to achieve a measure of stealth, while the F-117A compromised a lot of aerodynamic performance to achieve a much lower RCS. By the time the B-2A was being developed, computers were powerful enough to allow them to achieve maximum stealth with much fewer aerodynamic compromises. The F-22 achieves very high aerodynamic performance while still having a very low RCS. It reportedly isn't as low as the F-117A or B-2 but is still very good.