<a href="https://en.wikipedia.org/wiki/Bell_Aircraft" class="extiw" title="w:Bell Aircraft">Bell</a> <a href="https://en.wikipedia.org/wiki/Bell_X-1" class="extiw" title="w:Bell X-1">X-1</a> resting in the belly of a <a href="https://en.wikipedia.org/wiki/B-29_Superfortress" class="extiw" title="w:B-29 Superfortress">B-29 Superfortress</a>. [EDIT: the aircraft in the belly of the bomber is an X-1 of the first generation.]
Unknown, but listed as official Air Force Photo
Unknown date<div style="display: none;">Unknown date</div>, but taken in 1940s
Air Force Link <a rel="nofollow" class="external text" href="http://www.af.mil">site</a>. Provenance <a rel="nofollow" class="external text" href="http://www.af.mil/photos/index.asp?galleryID=159">page</a>, Image <a rel="nofollow" class="external text" href="http://www.af.mil/shared/media/photodb/photos/021002-O-9999G-005.jpg">page</a>