Formally and formerly, when indicating future action, first person used shall and second and third person used will; when indicating a strong desire, promise, or command, the two were flipped: first person used will and second and third person used shall. But this historical classification is now 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, we no longer have any good reason to delay its eventual retirement.