It's probably been in development for 2 1/2 years now. I'm actually already getting sick of the "It's just a Titanfall clone" remarks. Like Sledgehammer have been sat around doing nothing, and just waiting, until the Titanfall reveal last June and then they've said "right we've seen Titanfall now. Let's make a copy of it!" and suddenly they've made a game in 10 months People fail to understand that they'll have drawn up the ideas for this game LONG BEFORE Titanfall was ever shown.
In fact, how do we not know that Respawn had a spy in Sledgehammer's studio who's seen their ideas, passed them on to Respawn and copied them knowing their game would be out first and somebody said "we'd better chuck some mechs in so it doesn't look totally obvious"?