The "b" version was mostly the result of a desire for a cheaper version, and also the result of the several year delay after Carter canceled it.
The original version was a supersonic and low altitude bomber. Ths is a big technical challenge.
Carter cancelled it, and it went to sleep for a few years. during that time lots of other things advanced, the most important of which was stealth. The redesign backed off from supersonic low altitude requirement to just high subsonic. This made things cheaper due to things like fixed engine inlets. It added low observables technology; the B version is something like 1% of the radar signature of the A version. Backing off from supersonic also reduced the thermal signature. There were other changes, such as the escape pod on the A version that got cut back to ejection seats on the B version.