Not sure which B "form" you are playing, but I find it easiest to go from E to a full barre B. Just move the tip of the index up at an angle to the A string as it moves up two frets, getting the finger straight for the barre, and get that ring finger into position as you are moving.

Depending on the strum or picking you are doing, you could begin the transition as soon as you are down off the G string (G# note), finish off the open strings while you set up the B.