Due to the practical need for a deep wing, the flying wing concept is mostly adopted for subsonic aircraft. There has been continual interest in using it in the large transport role where the wing is deep enough to hold cargo or passengers. A number of companies, including Boeing, McDonnell Douglas, and Armstrong Whitworth, have undertaken design studies on flying wing airliners to date; however,[26] no such airliners have yet been built as of 2020.[citation needed]