Formally and formerly, when indicating future action, first person used shall, second and third person used will; when indicating a strong desire, promise, or command, the two were flipped: will for first person, shall for second and third person. Although this historical classification may still pass as formal, it is, in truth, little more than archaically quaint if not fully obsolete. Modern use has supplanted shall with will in almost all writing and speech, the only exceptions being interrogative suggestions (Shall we go?), imperative demands (You shall obey), and legal writing, where shall, according to some, implies duty—a vague and historically-nonsensical notion that is already on the decline.
As much as we may enjoy the fine air of a well-placed shall, and since it has no clear distinction from will, we have no good reason to delay its eventual retirement.