On Thanksgiving Eve, November 24, 1971 a man travelling under the name Dan Cooper hijacked a passenger airplane. He released the passengers in exchange for $200,000 and four parachutes. He then had the air crew take the plane back into the air, ordering them to fly to Mexico. At some point during the journey, he jumped out of the airplane with the money and parachutes. He was never found, and it is unknown whether he survived the escape.
The FBI questioned and then released a man by the name of Daniel B. Cooper, who was never considered a significant suspect. Due to a miscommunication with the media, however, the initials "D.B." became firmly associated with the hijacker and this is how he is now known.
Additional information: http://www.columbian.com/reflections/dbcoop.html