Bellybuttons would be the longest word I can think of beginning with a 'b' and ending in an 's'.
Ballerina is one 9-letter word beginning with B and ending with A.
A 4 letter word that begins with B is bend.
Buttercup is a 9 letter word. Bootstrap begins with b and ends with p.
jamb - The piece of the door frame that the door closes against.
that word will be BE EASY.