The U.S. Air Force is currently test-flying an optionally piloted supersonic drone over the Gulf of Mexico based on a modified F-16 fighter jet.

FORBES: Pilot-Optional Flight Is Closer Than You Think

That is critical for flying over America, where supersonic flight is banned because of the noise pollution created by sonic booms.

ECONOMIST: Business jets

The four current NASA pilots who can operate the modified 747 are ex-military aviators who split their time flying other planes including zero-gravity aircraft and T-38 supersonic jets.

MSN: What it's like to fly a plane carrying shuttle - Technology & science - Space | NBC News