BIPLANE is a 7 letter word that starts with B.

1. \Bi"plane\, n. [Pref. bi- + plane.] (A["e]ronautics)
   An a["e]roplane with two main supporting surfaces one above
   the other.
   
   

2. \Bi"plane\, a. (A["e]ronautics)
   Having, or consisting of, two superposed planes,
   a["e]rocurves, or the like; of or pertaining to a biplane;
   as, a biplane rudder.