It is off-the-beaten-path that development and the examination of the D-21 scout drone were done in a series of blackbird program. It was A-12 of the U.S. Air Force becoming the launching platform of the drone. Two airframe(serial number 60-6940 and 60-6941) was given the repair that is necessary for this duty , and it came to be called M-21. The flight of the combination of the M-21 mother machine and the D-21 drone that was called "Mother and daughter" was advanced up to an important stage of separating D-21. However, this plan was canceled because it had suffered a large number of casualties at the same time as losing the airframe due to the accident when the separation was examined. Moreover, though the experiment of the D-21 drone was continued afterwards, everything was discontinued along with the disposition of SR-71. Panel is not included.