Boeing 2707: the supersonic airliner that cost US$1 billion and never took off
Aircraft that intended to be bigger and faster than the Concorde spent the 60s in studies but was canceled in 1971 amid challenges related to noise and emissions
SST
Successor of the Concorde: bigger, but slower and with less capacity
If Lockheed Martin's proposal is correct, however, these new jets will be quiet and economically viable