The longest one that -I- know would be ballistocardiograph which is an instrument for detecting body movements caused by heartbeat.
Bellybuttons would be the longest word I can think of beginning with a 'b' and ending in an 's'.
Extraterritoriality is the longest word in the English language beginning with "ex."
Brunhilde is perhaps the longest. But that is just for English language.
Including the letters d, b, f, the longest word might be defibrillation.
xenoglossophobic which is the fear of foreign languages
extermination-y-ish.... i think.
A b word meaning helpful is beneficial.
21-letter wordbuckminsterfullerenes
pub
bauble
A b-word is any of a group of words beginning with B which may be considered rude or vulgar.
Another word for kindly beginning with 'b' is benevolent.