Northrop and Grumman merged to protect themselves from takeover attempts from Martin Mariettas
McDonnal Douglas offered itself up to Boeing for 'Merger' (buy-out) when they were by-passed on the JSF Phase 2 contract award.
Boeing had already acquired Rockwell International (B-1 & Shuttle) by that time.
Lockheed bought out General Dynamics & Convair and asumed their assets, then Martin Marietta
'merged' (fully acquired) Lockheed and dropped the 'Marietta' from their name by selling off their
aggregate and mining assets. Where 39 mighty independant Aerospace Giants once stood,
there were 3 Major players, and 2 smaller ones waiting for the next vulture to swoop down.
Incestuous families that eat their own.
If you really want to know
Kind of a bitter pill . . .