This plane flew out of the Johnson Space Center (originally the Manned Spacecraft Center) near Houston, Texas.  It would fly to the top of its climb, then the plane would be allowed to fall to the bottom of it path.  This exposed the occupants to a few seconds of no gravity (0-G).  It was used in Astronaut training and research experiments.  I managed this program for a year or so in the 1960s. The aircraft is now a mounded display at Ellington Air Base.
<a href="//commons.wikimedia.org/wiki/User:Jim_Evans" title="User:Jim Evans">Jim Evans</a>
<span class="int-own-work" lang="en">Own work</span>