In the late medieval system there were six normal notes, C D E F G A, and one note that had two forms, soft B (b) which was a semitone above A and hard B (♮) which was a whole tone above A. As written in the earliest sources, hard B looked a bit like an H with an added crossbar which may have been the reason for the change to H (or it was the next letter of the alphabet; both theories have manuscript support). Later, as RedGrittyBrick said, the soft b form was used to indicate any note which was a half-step above the previous note and hard b form for any note a whole step above the previous note (for instance F# would be written as F-natural, while an F-natural following F-sharp would be written as F-flat). The need for a third form, # (derived from the natural sign) only came later as notes could be seen as needing three different forms. Why hard B became the norm in some countries and soft B the norm in others is still an unexplained mystery, but might say something about how often B was performed flat or sharp in various countries.