On 16 March 2006, the Air Force announced that elements of the 131st Fighter Wing would become an associate unit assigned to the active duty 509th Bomb Wing. Consequently, the 131st Fighter Wing transitioned from flying and maintaining the F-15C Eagle fighter to the B-2 Spirit bomber. The final flight of the F-15C Eagle by the 131st occurred in June 2009 from Lambert International Airport in St. Louis. The unit was redesignated as the 131st Bomb Wing on 1 October 2008. The 509th and the 131st joined forces according to what is known as a "classic associate wing" structure. In a classic association, the active duty 509th retains ownership of the operational assets, including aircraft, maintenance facilities, and so on. However, each wing maintains its own chain-of-command and organizational structure, while the members of each unit perform their duties in a fully integrated manner. As a result, active duty and Air National Guard pilots and maintainers fly B-2 missions and sustain the aircraft as though they were one unit.