David S. F. Portree at Wired:
Although the fact is mostly forgotten today, NASA launched plans to replace the Space Shuttle even before the first Space Shuttle mission in April 1981. In 1985, President Ronald Reagan formalized these by signing a directive ordering the U.S. civilian space agency to develop a Space Shuttle successor. Notably, this occurred before the January 1986 Challenger accident laid bare the Space Shuttle’s frailties. NASA has attempted to develop a Space Shuttle successor ever since, but for a wide range of reasons it has not succeeded.
One of the early proposed Shuttle successors was called Shuttle II.