The two-issue 1981 arc from which the prequel-sequel is taking its cues is written by Chris Claremont and penciled by John Byrne, a most influential duo in the history of the X-Men. The story finds the X-Men visited by a time-traveling Kitty Pryde with dire warnings of a future timeline in which Xavier's team failed to stop Magneto and his Brotherhood of Evil Mutants from assassinating Senator Robert Kelly. As a result, Kitty's future is one in which most of the X-Men have been murdered and mutants reside in brutal internment camps.
Mark Millar, the comic writer whose work Vaughn adapted with Kick-Ass, was recently enlisted by Fox as a creative consultant on its stable of Marvel heroes. "Vaughn is going for a big sci-fi style thing with X-Men: Days of Future Past," Millar said in an interview with SFX via Total Film.
Describing the more obviously sci-fi time travel story, Millar said, "It's X-Men meets The Terminator."
Most importantly for fans, Millar alluded to the inclusion of the Sentinels, massive government-mandated mutant killing machines from the comics, saying, "You've got robots, you've got time travel, you've got superheroes - it's got everything in one film." A Sentinel appeared in silhouette in X-Men: The Last Stand, but only as an element of a Danger Room training session. The more grounded approach to the X-Men films has made the inclusion of Sentinels difficult, but the "what if" timeline shenanigans of Days of Future Past could create a plausible America featuring Sentinels.
Around the time of the X-Men: First Class release, Vaughn shared his idea to incorporate the assassination of John F. Kennedy into the sequels story, with Magento controlling the bullet. That notion may very well find its way into the sequel, given that an assassination is a crucial component of the comic book storyline.
Vaughn's currently working with Simon Kinberg on the script, preparing for a production scheduled to begin early next year in order to meet a July 26, 2014 release date. The principal cast from First Class, including Michael Fassbender, James McAvoy, Jennifer Lawrence, Nicholas Hoult, and Caleb Landry Jones, are expected to return for another round of mutant action.