The Bb flute, also called an alto flute is pitched below the common D or concert pitch flute and is bigger in size but not so big as to be unmanagable. It has a much different sound to the D instrument or the brighter Eb flute having a deeper more sonorous feel to it.
Because of its larger size the pipers technique of using the flats of the fingers on the lower three finger holes makes the fingering easier. The top three finger holes can be fingered with the tips of the fingers in the standard way. The technique of sounding the flute is perhaps much more from the diaphragm.
My Bb flute is based on Rudall, Rose & Carte instruments with adjustments to finger hole size and the upper bore.
It is available without tuning slide.