In an interview with IGN, producer Bryan Singer confirmed that the sequel to X-Men: First Class will indeed be titled X-Men: Days of Future Past. Longtime comic book fans know that this is also the title of a watershed story arc in the X-Men universe, which was written by Chris Claremont and illustrated by John Byrne in 1981.
The basics of the original story revolve around an alternate dystopian future where mutant-hunting Sentinel robots rule the U.S. and mutants are either sent to concentration camps or killed on sight. There are other specifics to the story that we're not sure will make it into the film, but Singer still promised that this will be an "ambitious" undertaking.
Interestingly enough, Singer also hinted that this X-Men movie could have ties to the previous ones that he directed in the early 2000's. If the time-traveling and alternate timeline aspects of the original "Days of Future Past" comic are intact, it's possible that this movie could incorporate all sorts of characters and plot threads from the prior X-Men films as well. This could wind up being very similar to the way J.J. Abrams' Star Trek reboot attempted to merge the various timelines from that series into a new continuity.
There is a lot to speculate so far, but we won't know anything for sure until X-Men: Days of Future Past hits theaters on July 18, 2014.