AP reports that Enterprise was NASA's first shuttle, built in 1976:
Of the six shuttles that NASA built, only four remain: Enterprise plus Discovery, Atlantis and Endeavour. The other two experienced disasters during their missions, killing their crews: Challenger exploded in 1986 and Columbia disintegrated in 2003.
President Richard Nixon first announced NASA's intention to build the shuttle fleet in 1972 amid heightened tensions during the Cold War. With U.S. officials fearing the Soviet Union would dominate the novel realm of space travel, the shuttle was designed to be a reusable spacecraft that could also land safely — allowing NASA to conduct launches more frequently and with greater efficiency.