The B-1B - the original B-1A project was cancelled - went into operational use in the mid-1980s but was not used in anger until December 1998, against Iraq.
For example, when the B-1A program was restarted to produce the B-1B, the per unit cost of the B-1B was three-times higher than the B-1A. Importantly, the B-1B was a modification (albeit a significant one) of the A-model and a gap of just 18 months occurred in the production line.