Assigned to the 393rd Bomb Squadron, 509th Composite Group, it was used as a weather reconnaissance plane and flew to the city of Nagasaki, designated as the secondary target, before the final bombing to determine if conditions were favorable for an attack. Crew B-8 (Capt. Charles McKnight, airplane commander), normally assigned to another aircraft, actually flew the mission.
On July 27, 1945, Costello and his crew flew the airplane from Wendover to Kirtland Army Air Field, Albuquerque, New Mexico, accompanied by another 509th B-29 and one from the Manhattan Project test unit at Wendover (216th Base Unit). There each loaded one of three Fat Man atomic bomb assemblies (without the plutonium core, which had left the day before by courier on one of the 509th CG's C-54 Skymaster transports) in its bomb bay for conveyance to Tinian.
The three bombers flew to Mather Army Air Field, California, on July 28, and took off for Hawaii on July 29. Shortly after takeoff from Mather, a panel door on Laggin' Dragon enclosing the life raft compartment opened and ejected the raft, which wrapped around the vertical stabilizer and impeded the B-29's rudder. The aircraft nearly crashed in the Pacific Ocean but the pilots managed to return safely to Mather. After minor repairs, Laggin' Dragon and its cargo continued to Hawaii, finally reaching Tinian on August 2.
It was assigned the square P tail identifier of the 39th Bomb group as a security measure and given victor number 95 to avoid misidentification with actual 39th BG aircraft. The airplane was named and nose art applied after the atomic missions, and purportedly referred to the methodical Costello's reputation for "lagging behind" and the 393rd Squadron's dragon insignia. It arrived too late to participate in other combat operations and participated in two practice flights subsequent to the atomic attacks.
The airplane returned to the United States in November 1945, based with the 509th CG at Roswell Army Air Field, New Mexico. In June 1946 it was part of the Operation Crossroads task force based on Kwajalein. In June 1949 it was transferred to the 97th Bomb Wing at Biggs Air Force Base, Texas, and in April 1950 was converted to a TB-29 trainer at Kelly Air Force Base, Texas, and the Oklahoma City Air Materiel Area at Tinker Air Force Base.
It was subsequently assigned to:
Crew B-8 (regularly assigned to Top Secret)