At the bottom of the page I linked to above, there is a link to a profile of the Berlin Express. Including modeling tips from the man himself. The profile there shows him having 2 kills.
The web-site http://www.acesofww2.com/USA/ credits him with 3. I was unable to find him listed on other aces websites I looked at.
He tells an amusing story about running into some Luftwaffe and the German pilots just running away. One pilot was apparently so scared of getting shot down that he just bailed out. Bill was offered credit for the "kill" because he was the closest to him at the time. The story is funny, because he had left ammo behind on the ground because they got a hold of some beet vodka (which they preferred to potato vodka) and he couldn't bare to leave it behind.
He refused credit for the kill, but maybe some lists are giving him credit for it?