In announcing the May 20 release of Wolfenstein: The New Order, Bethesda also outed the new Doom game (formerly referred to as Doom 4). What's more, pre-ordering Wolfenstein: The New Order will grant you access to Doom's beta.
Not much is known about the next installment in the Doom series, nor do we know when the beta will actually begin. Bethesda only confirmed that inside of your boxed copy of Wolfenstein: The New Order will be a key code which, once redeemed, will allow you to participate in the Doom beta once it begins.
Bethesda hasn't yet announced when the beta will begin, but prior to its start Bethesda will email pre-order customers with additional info.