The future tense in standard written English uses 'will' as the verbal modifier, but increasingly English speakers use 'going to' as the verbal modifier. When did this come into common usage?
'I am going to swim the English Channel' is a way of expressing a future intention in English. And whether we call it a 'future tense' or not 'I will swim the English Channel' is the more direct way of saying almost exactly the same thing about one's future plans.
This corresponds with an almost identical situation in French. And I am wondering if that is the origin of the English form.
'Je vais nager à travers la Manche' literally means 'I am going to swim the English Channel'.
'Je nagerai à travers la Manche' literally means 'I will swim the English Channel'.
The English use of 'going' in this way is confirmed by the OED as meaning V 47 b, of the verb 'go'. No etymology of this use is given except that it is compared to the French equivalent as i have done above.
In French (please let's not start a ruckus over my French like last time), as young children, we learn an easy future construction Je vais (I am going) plus an infinitive (say, aimer, to like/love) as a simple way to construct a future tense. J'aimerai is a bit harder.
It's reallyvery similar to learning English as children. Children learn -ed is a past tense for every verb. (I dropped, I wanted, I runned), at which point they learn irregular verbs, etc. through repeated exposure. My mother never sat me down and stated, "Today you will learn to conjugate run. I run, you run, he/she/it runs..." Children keep it simple until they know better. It's the weak conjugation first.
Perhaps that is when I'm going to (verb) (short: I'm gonna) came into our language - with the Norman Conquest of 1066.
Go appeared before the conquest (before 900; Middle English gon, Old English gān), going ~1250–1300; the -ing construction is Middle English (-ing, -inge ; the variant -in). I don't have an OED susbscription, nor the expertise of linguists, so I should stop here.