They do have different pronunciations, it's just than most speakers have lost the distinction. B is plosive and V is fricative.
Some regions still make that difference, at least senior people do. I had a language teacher from old Castille who sound her spelling of v and b very clearly. She made the distinction between 'll' and 'y' clear as well, with 'll' sounding as a modified 'l', a cross between 'li' and 'ye'.