I dunnoh how real that distinction has ever been. I read stuff from 1800, or Shakespeare, and I just don't see the shall/will distinction. Is it really there?
I wouldn't be amazed to find this is something Victorian grammarians thought up.
From
etymonline:
shall : O.E. sceal "I owe/he owes, will have to, ought to, must" (infinitive sculan, pt. sceolde), a common Gmc. preterite-present verb, from P.Gmc. *skal-, *skul- (cf. O.S. sculan, O.N., Swed. skola, M.Du. sullen, O.H.G. solan, Ger. sollen, Goth. skulan "to owe, be under obligation;" related via past tense form to O.E. scyld "guilt," Ger. Schuld "guilt, debt;" also O.N. Skuld, name of one of the Norns). Ground sense probably is "I owe," hence "I ought." The sense shifted in M.E. from a notion of "obligation" to include "futurity." Its past tense form has become should (q.v.). Cognates outside Gmc. are Lith. skeleti "to be guilty," skilti "to get into debt;" O.Prus. skallisnan "duty," skellants "guilty."
will (v.) : O.E. *willan, wyllan "to wish, desire, want" (past tense wolde), from P.Gmc. *welljan (cf. O.S. willian, O.N. vilja, O.Fris. willa, Du. willen, O.H.G. wellan, Ger. wollen, Goth. wiljan "to will, wish, desire," Goth. waljan "to choose"), from PIE *wel-/*wol- "be pleasing" (cf. Skt. vrnoti "chooses, prefers," varyah "to be chosen, eligible, excellent," varanam "choosing;" Avestan verenav- "to wish, will, choose;" Gk. elpis "hope;" L. volo, velle "to wish, will, desire;" O.C.S. voljo, voliti "to will," veljo, veleti "to command;" Lith. velyti "to wish, favor," pa-vel-mi "I will," viliuos "I hope;" Welsh gwell "better"). Cf. also O.E. wel "well," lit. "according to one's wish;" wela "well-being, riches." The use as a future auxiliary was already developing in O.E. The implication of intention or volition distinguishes it from shall, which expresses or implies obligation or necessity. Contracted forms, especially after pronouns, began to appear 16c., as in sheele for "she will." The form with an apostrophe is from 17c.