The Boeing-designed B-29 S/N 44-27297 was built by the Glenn L. Martin Co. at Omaha, Neb., at a cost of about $639,000. It was accepted by the USAAF on April 19, 1945, and was delivered to the 393rd Bomb Squadron at Wendover Field in the Utah salt flats. There aircrews of the 509th Composite Group were engaged in intensive training under a cloak of secrecy. In June, aircraft and crew flew to Tinian Island in the Marianas. From there, Bockscar, named for its pilot Frederick C. Bock, flew five bombing missions. On four of these, a 10,000-pound bomb loaded with high explosives was dropped.