>>9653383
(LB)
>As such, using a 737 to test a new subsystem that is destined for an aircraft like the B-21 would be right within their purview.
This is the conclusion I drew from that article. A flying testbed craft, for a sub-system such as a radar or communications array. You would test it at altitude under flight conditions so you could troubleshoot the whole system prior to installing in a new (and maybe unavailable) airframe.