I asked this question on the BB-8 Builders Club forum and got this response. I don't know if it is accurate, but -
BB is the moniker for Bothan Ball Droids, located on a secretive Homeworld few have ever seen. Rumors swirl about a nearly magical high tech manufacturing facility where the BB series is produced, but left inactivated until delivery. The units are powered on by their ordering purchaser and do not have any way to identify the location of their manufacturer. Repair sites are scattered across the Outer and Inner Systems in case of a need for parts.
The first few series of BB droids were not as successful but do make appearances in the Star Wars saga. For example, the black hemispherical BB-1 series droid in the sand crawler scenes on Tatooine, the flying BB-1.5 light saber trainer drone, and the far more sinister BB-2T Imperial Torture droid (which was a design iteration from the light saber training droid, rumored to be the last development cycle by the previous leadership of Bothan Ball before an internal build team staged a coup using their own iteration of BB technology).