Was the original purpose of the B-1 to attack the Soviet Union with nuclear weapons?
I don't think they every worked out the EW gear to the level that it could penetrate airspace defended by Radar guided weapons.
Tactically, the B-1 really offers few advantages over the B-52.
The original purpose of the B-1 Lancer begun in 1967, was to provide a source of government support for the R&D necessary to develop America's Supersonic Transport(SST), the Boeing 2707, which was a spitting image of the Lancer.
Many people don't recall that the competition for the SST between US, European consortium and the USSR was a big political football back in the day. President Nixon was all for the SST but ultimately it was seen for the wasteful boondoggle it was, Congress denied funding, and the US SST was never built.
The Russians built the TU-144, which fell apart in flight, and the EU built the Concorde, which was a commercial failure.
One of those rare moments in history when Congress made the right decision.