The following is from the Oct. 22, 2013 Congressional Research Service report: U.S. Strategic Nuclear Forces: Background, Developments, and Issues.
Even though the United States plans to reduce the number of warheads deployed on its long- range missiles and bombers, consistent with the terms of the New START Treaty, it also plans to develop new delivery systems for deployment over the next 20-30 years. The 113th Congress will continue to review these programs during the annual authorization and appropriations process.
During the Cold War, the U.S. nuclear arsenal contained many types of delivery vehicles for nuclear weapons. The longer-range systems, which included long-range missiles based on U.S. territory, long-range missiles based on submarines, and heavy bombers that could threaten Soviet targets from their bases in the United States, are known as strategic nuclear delivery vehicles. At the end of the Cold War, in 1991, the United States deployed more than 10,000 warheads on these delivery vehicles. That number has declined to less than 2,000 warheads today, and is slated to decline to 1,550 warheads by the 2018, after the New START Treaty completes implementation.