The B-1B began operations in 1986 as a long-range heavy bomber designed primarily to carry nuclear munitions. Although the B-1B's nuclear mission was withdrawn in October 1997, the Air Force continues to rely on the B-1B to support conventional wartime missions. The B-1B has the largest payload of the Air Force's three bombers, and recent modifications have provided the capability to deliver near precision munitions. Future upgrades to the B-1B are expected to provide greater flexibility by enabling it to carry three different types of bombs simultaneously and eliminate some of its long-term survivability and maintainability problems by improving its radar warning systems, jamming ability, and other electronic countermeasures. In May 2001, the Office of the Secretary of Defense suggested retiring the entire B-1B fleet by October 2001. In June 2001, the Air Force proposed an alternative that reduced the B-1B fleet from 93 to 60 aircraft and consolidated them at two active duty locations instead of the three active duty and two National Guard locations that housed the aircraft. Congress delayed implementation of the fleet reduction until the Air Force completed a review of bomber force structure and provided a report on alternative missions and basing plans. The Air Force began consolidating the fleet in July 2002. GAO found that Air Force officials did not conduct a formal analysis to assess how a reduction in B-1B bombers from 93 to 60 would affect the Department of Defense's ability to meet wartime requirements. Nor did they complete a comprehensive analysis of potential basing options to know whether they were choosing the most cost-effective alternative. A comparison of active and Guard units' missions, flying hour costs, and capabilities showed that active and Guard units were responsible for substantially the same missions but Guard units had lower flying hour costs and higher mission-capable rates than their active duty counterparts. Additionally, the Guard's B-1B aircrew members were generally more experienced, in terms of the number of hours flown, than the active duty B-1B aircrews because most Guard aircrew members served on active duty prior to joining the Air National Guard.