Old English will, willa, willan, wyllan, from Proto-Germanic *weljon (cf. Old Saxon willio, Old Norse vili, vilja, Old Frisian willa, Dutch wil, willen, Old High German willio, wellan, German wille, wollen, "will") related to *willan "to wish". Gothic wilja,waljan "to choose" Avestan verenav- "to wish, will, choose;" Greek elpis "hope;" Latin volo, velle "to wish, will, desire;" Old Church Slavonic voljo, voliti "to will," veljo, veleti "to command;" Lithuanian velyti "to wish, favor," pa-vel-mi "I will," viliuos "I hope;" Welsh gwell "better" )Indo-European *wel-, meaning to wish for or desire.)
These all words are originated by Sanskrit vi-ln-gh-n foya?ku meaning "pass over or beyond, overcome, surpass, go over, excel, transgress". All the meaning show 'ahead, forward, onward' which indicate toward 'future'.
Early Germanic did not inherit any proto-Indo-European forms to express the future tense, and so the Germanic languages have innovated by using auxiliary verbs to express the future (this is evidenced in Gothic and in the earliest recorded Germanic expressions). In English, shall and will are the auxiliaries that came to be used for this purpose. (Another one used as such in Old English was mun, which is related to Scot maun and Modern English must, which is based on Sanskrit mu-n* eq.k means 'to promise'.)