Xbox One owners who pre-order Fallout 4 through the Xbox Store now can get Fallout 3 free within 7-10 days, Bethesda's vice president of PR and marketing Pete Hines told Polygon this morning.

The free game offer isn't limited to Xbox Store pre-orders, but the timing is. To claim your free Fallout 3 copy, there are two scenarios, depending on where and when you pre-order Fallout 4.

Pre-order Fallout 4 as a download through the Xbox Store, and Microsoft will send you a code within 7-10 days to download The Xbox 360 version of Fallout 3 . It will be playable immediately on that system and at an undetermined date when the game becomes backward compatible on Xbox One. "I don't know what the timeframe is as to when that will happen," Hines said.

Pre-order Fallout 4 at retail, and you'll receive the Fallout 3 download code when you pick up your physical copy of Fallout 4 on its Nov. 10 release.

It splits like this, Hines said, because you'll pay $59.99 upfront for Fallout 4 when you pre-order it through the Xbox Store, while you might not even have to put money down for a retail pre-order.

Hines said that he isn't sure how long the "limited time" Xbox offer will last, though he did explain the reasoning behind it.

"Basically it's meant to make sure folks know that this is not a 'forever and ever' thing," he said. "So that does mean there is a scenario where you could go into a store to pickup a copy of Fallout 4 post-launch and not get a copy of the game with the code. But I'm not sure how many copies will include it, or for how long. To be determined."

Hines also told Polygon that the offer is exclusive to Xbox and will not apply to other platforms like PlayStation and Steam.

Bethesda detailed Fallout 4 at its first E3 press conference. The next day at Microsoft's E3 2015 press conference, game director Todd Howard took the stage talk more about the game. As he was speaking, a video overlay appeared in the livestream. It read, as you can see below, "Xbox exclusive offer: Fallout 3 included with Fallout 4."

The pre-order requirement, however, wasn't mentioned.

At the same press conference, Microsoft announced that the Xbox One will be backward compatible with Xbox 360 games. About 20 games are already working, and the company plans to support 100 by this holiday season.

Developer Bethesda Game Studios' Fallout 4 is set for release on PlayStation 4, Xbox One and Windows PC. To ease the wait until you enter Vault 111, you can press play below and watch its first trailer.