Lockheed Martin tested the F-35's gatling gun a few weeks ago. It's called the GAU-22/A, it can fire 3,300 25mm rounds per minute and it is more precise but not as powerful as the car-sized A-10's GAU-8 Avenger.
While the GAU-22/A has been successfully tested, the weapon won't be ready for combat till 2019.
The reason: They are still developing its control software.