Rumored earlier this week, the sequel to 2011's X-Men: First Class is officially titled X-Men: Days of Future Past. IGN scored a confirmation directly from the film's producer, Bryan Singer.
"It's called 'Days of Future Past'," he told the site. "It deals with aspects of that comic, but also some very new things."
The comic book storyline, "Days of Future Past" ran in Uncanny X-Men #141 and 142 way back in 1981 during Chris Claremont and John Byrne's run and introduced the idea of an alternate future for Marvel's mutants that grew out of the Brotherhood of Evil Mutants killing an important senator, leading to a future where all mutants are hunted by Sentinels.
As you can see in the video player below, Singer also says that the hope is to expand the X-Men franchise to become as complete a world as the interconnected film universe of Marvel's The Avengers.