Since there was still no official reaction from Japan, the Americans felt that there was no alternative but to prepare a second atomic attack. The plutonium bomb called "Fat Man" was loaded into a B-29 known as "Bockscar" (Martin-Omaha built B-29-35-MO serial number 44-27297, the name often spelled Bock's Car), named after its usual commander, Captain Frederick C. Bock. However, on this mission, the aircraft was flown by Major Sweeney, with Capt. Bock flying one of the observation planes. The primary target was to be the Kokura Arsenal, with the seaport city of Nagasaki as the alternative.