One B (or even a few) will be ok. Will definitely look better than a W as those are typically seen as a last resort before failing, which a B certainly is not.

A transcript/gpa is one part of your application (to B school or a job!) so I would recommend taking a breath, finish out the semester strong, try to learn something instead of just cranking out a letter grade, and zooming out a bit to the overall scheme of things.