Just made a B body for my C whistle: A standard C whistle head with B body.
Worked out very nice, a sweet sound with very good second octave behaviour, thanks to the relative narrower bore (higher length to bore ratio). Listen to a sound sample, just something which came into my head:
http://soundcloud.com/hans-bracker/b-whistle-sample
There is something really beautiful about playing a B whistle, I can't describe.
My theory is that what we know as modern B was once in olden times C, and what we know as Db/C# was in olden times D, because the general pitch has been raised.
C whistle with extra B body will make a good combo set.