If we want to get down into strict definitions, a "D" bow is one with a "D" shaped cross section to the limbs. Flat on the back of the bow and rounded on the belly (e.g., an English longbow). There aren't too many custom makers of this type.
It's only been fairly recently that the term "D" bow has applied to a bow whose shape when strung resembles the letter "D" which would include a flatbow, American Semi Longbow (ASL), "Hill Style" bow, or some mild r/d bows. There's lots of custom makers of this type.