For what it is worth..................
"The B-21 designation makes a lot of sense marketing-wise but may upset the most extreme military aviation aficionados out there. The “21" in B-21 is clearly there for marketing purposes, signifying that it is the bomber of the 21st century. It is also interesting that the first lot of production aircraft will include 21 units.
The B-21 designation deviates from the USAF’s modern bomber designation system that has included B-1 and B-2, with B-3 being the predicted designation for the LRS-B and the failed Next Generation Bomber and 2018 Bomber initiatives that came before it.
The truth is the Pentagon’s aircraft designation systems have never made cohesive sense. The B-21 is named so because of marketing, especially to lawmakers who control the purse strings on Capitol Hill, and the young but essential program needs all the help it can get."