Announced as part of the Activision Blizzard Q3 results
http://investor.activision.com/releasedetail.cfm?ReleaseID=939963
Blizzard Entertainment began closed beta testing for Overwatch on October 27, 2015, with over 7 million players signed-up to participate, not including China.
http://investor.activision.com/releasedetail.cfm?ReleaseID=939963