For your information the GAM-77/AGM-28 Hound Dog Missile was used on both the B-52G & H models.  I was a missile mechanic on the Hound Dog for both of these models of the B-52.  While the Skybolt Missile never became operational it was also launched from the B-52H model on April 20, 1962 over the Atlantic Missile Test Range.  Each B-52H was supposed to carry four Skybolt Missiles, two on each wing pylon.  The X-15 was much earlier and would have been launched from one of the earlier models of the B-52.