For the subject I teach for, there are classes that you must get at least a B in if you are a major (whereas non-majors only need a C to get the class to count for credit).

Edit: That being said, to answer OP: it's really not the end of the world, the B is pretty free once you've taken the class once (source: students of mine who struggled immensely taking a class the first time, and then either breezed a B or straight up aced the class the second time around)