Historically speaking, modal auxiliaries are either (for today's purposes) present-tense or past-tense forms 'can-could, may-might, shall-should, will-would', 'ought' is apparently an obsolete past-tense form of 'owe', and I'm not too sure what 'must' is (or was). However, grammarians' inexplicable invention of "future tense" for a future-tenseless Germanic language like English seems all the more mysterious when the example of German stares us clearly in the face. German has obvious "modal infinitive" cognates to the modals of English: 'can-können, may-mögen, must-müssen, shall-sollen, will-wollen', all of which (except 'mögen' usually) may use their present-tense forms for future time reference.
Kann sie es morgen tun? (Is she able to do it tomorrow?)
Er soll es morgen tun. (He is supposed to do it tomorrow.)
Du musst es morgen tun. (You must do it tomorrow.)
Ich will es morgen tun. (I want to do it tomorrow.)
It's interesting to note that German 'wollen' is still used for "volition" rather than strict "futurity", as the English main verb and noun 'will' still is.
He willed the spoon to bend.
It's a matter of willpower.
Indeed, as auxiliaries 'will-would' still very much have a "volition"-related use, as mentioned by RonBee.
George will always whistle tunelessly in the bath.
George would always whistle tunelessly in the bath until poor old Myrtle started putting arsenic in his rum and coke.
I wish you wouldn't do that!
As a matter of interest, where German uses an auxiliary that translates directly as 'will' into English it uses 'werden', which as a main verb means "become".
Er wird dick. (He's getting fat.)
Er wird dick werden. (He'll get / He's going to get fat.)
Calling 'will' "future tense" has created a lot of confusion, since students often seem to fall for the canard that 'will' is future and future is 'will', when nothing could be further from the truth, and commonly use it erroneously as their future-reference default form.
Be all this as it may, the past of main verb 'will' is 'willed', and the past of present-tense auxiliary 'will' is 'would'.