In Viz's dubbed version of Bleach - episode 42, Yourichi herself explains the qualifications of becoming a captain of the court guard.
Yoruichi - "before any Soul Reaper can become a captain he or she must have mastered both of these Zanpakutō releases (shikai and bankai)," she goes on to say "...except for one (Kenpachi Zaraki) every squad captain of the court guard has always had to first master benkai. " Yoruichi gets more specific when she says "...As far as I know in all the vast history of the Soul Society Kenpachi is the only one to ever to become captain with out first achieving bankai..."
Since Yoruichi was the former "2nd division captain" she has a sword and has achieved shikai and bankai. All the the proof is right there in episode 42. This means Soi fon herself has a bankai release too, and the same holds true for Isshin.
Let me hear if you agree or not?