Uncanny X-Men, Vol. 1 #141 (1981)

Days of Future Past
Thirty years from now, the world will be an ugly place. Sentinels have been reactivated, and nearly all superbeings have been hunted and killed. Very few remain, and they try an incredibly risky plan: they send Kitty Pryde's mind back to her 13 year old body so she can prevent the assassination of Robert Kelly, Charles Xavier and Moira MacTaggert, ultimately averting the future she comes from.
Release Date: 1981