The anticipated follow-up to the surprise X-hit will take cues from the iconic X-Men story arc, originally published in 1981 in the pages of Uncanny X-Men #141 and #142,
The story involves time travel and a dystopian vision of the X-Men's future. Following the Brotherhood of Evil Mutant's assassination of an anti-mutant US Senator, mutantkind (and, in the comics, most other non-mutant superhumans like Captain America and Spider-Man) are hunted down and killed by giant robotic Sentinels. The X-Men of the present day must fight to change the future and save their species.
Producer (and former X-Men mainstay) Bryan Singer confirmed the news to IGN earlier this week, going on to say that the adaptation "deals with aspects of that comic, but also some very new things."
This makes sense, a lot of the core elements of Days of Future Past would not work using the established First Class timeline (which is standing on shaky ground, continuity-wise, as it is), as a lot of the core characters in the story will not be introduced into the movie universe until the original X-trilogy.
As a result, the adaptation will be forced to lose its most iconic image: That of Wolverine being disintegrated, leaving only an adamantium skeleton behind. But this is a worthy sacrifice if it means we don't have to see Halle Berry's take on Storm again.