The definition of B movie, in this thread, is rather inconsistent.
Well, the classic definition is a film that was made to either supplement a Feature Film, or one that was never meant to be a Feature Film. It was something to show along with the "real" movie.
In the old days it was often a mechanism to satisfy actor contracts, back when actors signed exclusively with a movie company. An actor would sign for X number of pictures, and the studio had to satisfy it or be in breach of contract, allowing them to switch movie companies. Often the quality of these films, and their budgets, reflected this.
Sometimes one gets
The Quiet Man, which was a two-fer contract filler for John Wayne and Mareen O'hara for doing
McLintock! (and was in fact a Feature Film), but other times one gets
The Three Musketeers, where John Wayne's portrayal of an Arab is amazingly identical to every other role he performed.
Indeed, today movie companies will make films using intellectual properties that are never released to satisfy their contractual obligation to the owners in order to keep them.
Today we can safely define a "B" movie as one that either had a very limited release in theaters or never meant to be shown at theaters. Anything direct-to-DVD qualifies, as do most "made for TV" movies.