In 1992, McDonnell Douglas unveiled a study of a double deck jumbo-sized aircraft designated MD-12.  Despite briefly leaving the market, the study was perceived as merely a public relations exercise to disguise the fact that MDC was struggling under intense pressure from Boeing and Airbus. It was clear to most in the industry that MDC had neither the resources nor the money to develop such a large aircraft, and the study quickly sank without a trace. A similar double deck concept was used in Boeing's later Ultra-Large Aircraft study intended to replace the 747, but ultimately the double deck concept would not see the light of day until the Airbus A380 in the 2000s.

Following Boeing's 1996 acquisition of Rockwell's North American division, McDonnell Douglas merged with Boeing in August 1997 in a US$13 billion stock-swap to create The Boeing Company.

Source: Wikipedia