The museum has many rare and important aircraft and other exhibits , including one of four surviving Convair B-36s , the only surviving XB-70 Valkyrie , and Bockscar -- the B-29 Superfortress that dropped the second atomic bomb in World War II . Included in the permanent collections are one of four surviving Convair B-36s , the only surviving XB-70 Valkyrie , and Bockscarâ '' the B-29 Superfortress that dropped an atomic bomb on Nagasaki during the last days of World War II .