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"The B-1A was initially developed in the 1970s as a replacement for the B-52. Four prototypes of this long-range, high speed (Mach 2.2) strategic bomber were developed and tested in the mid-1970s, but the program was canceled in 1977 before going into production."

By this guy in his first year in office:

Canceling the B-1A was not a bad decision. From Wikipedia's B-1 entry:

In 1976 Viktor Belenko defected to Japan with his MiG-25 Foxbat, and described a new "super-Foxbat" that had look-down/shoot-down radar systems in order to attack cruise missiles. This would also make any low-level penetration aircraft "visible" and easy to attack. Countering this problem would require another upgrade to the electronics suite, already one of the most complex and expensive ever fitted. The debate over the need for the bomber opened anew, and this time the reduced low-speed dash was a particular target. Given the performance and the armament suite that was similar to the B-52, the program was increasingly questioned as a very expensive solution that appeared to have limited benefits over the existing fleet.

When President Carter took office in 1977 he ordered a review of the entire program. He was informed of the relatively new work on stealth aircraft that had started in 1975, and decided that this was a far better avenue of approach than the B-1. On 30 June 1977 he announced that the B-1A would be cancelled, in favor of ICBMs, SLBMs, and a fleet of modernized B-52s armed with ALCMs.[3] No mention of the stealth work was made public, the program being top secret, but today it is known that he started the Advanced Technology Bomber (ATB) project in early 1978, which eventually led to the B-2 Spirit.[8]

...

On taking office, Reagan was faced with the same decision as Carter before; whether to continue with the B-1 for the short term, or to wait...Numerous changes were made to the design to better fit it to real-world missions, resulting in the new B-1B.