I think you are a little confused. The A-12 ATA was designed by McDonnell Douglas and General Dynamics. It was cancelled before the prototype was completed. Lockheed's predecessor to the SR
-71 was also called the A-12 and 18 were built and flown.
Since 1970, the Skunk Works has built and flown the Have Blue prototypes, the F-117, the YF
-22 prototypes, the X-35 prototypes, the Darkstar UAV, JASSM and recently, an airship. They have also reengined the U-2Rs, making them U-2Ss.