N661US

N661US is an aircraft that was built by Boeing as the prototype of the Boeing 747-400, a modernized version of the Boeing 747. The plane rolled off the assembly line on January 26, 1988, and had its first flight on April 29, 1988. After the 747-400 flight testing program had concluded, Boeing delivered the aircraft to Northwest Airlines on December 8, 1989.

In 2002, the 747 was involved in an incident in which it experienced a lower rudder hardover event. This occurs when an aircraft's rudder deflects to its travel limit without crew input. The pilots were able to overcome the issue and land safely. The problem was blamed on metal fatigue, and an airworthiness directive was issued to prevent the possibility of a future accident.

The aircraft was eventually transferred to Delta Air Lines, after Delta's merger with Northwest in 2009. While in service with Delta, it was known as Delta Ship 6301. It continued in passenger service until it was retired on September 9, 2015. Later, it was transferred to the Delta Flight Museum in Atlanta, Georgia, where it remains on display.