Every single Xbox One can be used to make games, Microsoft says—in other words, you can use the next-gen console as your very own development kit.
The Xbox 360 was a closed system, but it appears that its successor will be far more open. In a statement to Kotaku today confirming news that leaked earlier on Game Informer, Xbox's corporate vice president Marc Whitten said that all Xbox Ones will function as development kits, which are usually only available to licensed developers.
Here's Microsoft's official comment:
Our vision is that every person can be a creator. That every Xbox One can be used for development. That every game and experience can take advantage of all of the features of Xbox One and Xbox LIVE. This means self-publishing. This means Kinect, the cloud, achievements. This means great discoverability on Xbox LIVE. We'll have more details on the program and the timeline at gamescom in August.