After training, the unit was reassigned to the South West Pacific theatre in late in 1944 for service with the Twentieth Air Force. The 504th was assigned to the 313th Bombardment Wing on Tinian. The group began combat operations from Tinian in Jan 1945 with attacks on Japanese airfields and other installations on Maug and Iwo Jima and in the Truk Islands. Flew its first mission against the Japanese home islands early in Feb 1945 when the group bombed the industrial area of Kobe. Continued to attack strategic targets in Japan, operating in daylight and at high altitude to bomb such objectives as aircraft factories, chemical plants, harbors, and arsenals.
Received a Distinguished Unit Citation (DUC) for striking the industrial center at Yokohama late in May 1945. Began incendiary raids in Mar 1945, flying at night and at low altitude to strike area targets in Japan. Started mining operations against enemy shipping late in Mar, receiving a 2d DUC for mining Korean shipping lanes, the Shimonoseki Strait, and harbors of the Inland Sea, July-August 1945. In April and May 1945 the group hit airfields from which the Japanese launched kamikaze planes against the invasion force during the assault on Okinawa. During mid June 1945 the 680th Bomb Squadron joined the 504th bomb group bring them up to full strenght of 3 squardons.
After the war it dropped food and supplies to Allied prisoners, participated in show-of-force missions, and flew over Japan to evaluate damage inflicted by bombardment operations. Moved to the Philippines in March 1946. Inactivated at Clark Field on Luzon on 15 June 1946.
Historical note: The 504th Bombardment Group flew the last combat mission by the United States Army Air Forces of World War II, its last combat mission being on 15 August, 1945.