[mulmen]

Boeing (later United Aircraft) used to operate their own airline (later United Airlines). They also bought Pratt & Whitney, Hamilton Standard, Chance Vought, National Air Transport, and Pacific Air Transport. They were vertically integrated across the airline industry by the end of the 1920s.

This all culminated in the Air Mail Act of 1934 [1] which prohibited airlines and airplane manufacturers from operating under the same corporate umbrella.

It will be interesting to see where this ends up with SpaceX integrating the consumer facing airline operation with rocket manufacturing. Something commercial airlines are not able to do.