The unit was established in early 1944 at Dalhart Army Air Field, Texas, being formed as a Boeing B-29 Superfortress very heavy bombardment group.

The 393d would eventually become the only military unit in history to engage in nuclear warfare, dropping Atomic Bombs on Hiroshima and Nagasaki, Japan, in August 1945.

As a two squadron group, the 504th was deployed to Pacific Theater of Operations in late 1944, being assigned to the XXI Bomber Command 313th Bombardment Wing in the Northern Mariana Islands; being stationed at North Field, Tinian.

The group began combat operations from Tinian in January 1945 with attacks on Japanese airfields and other installations on Maug and Iwo Jima and in the Truk Islands.

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.

After V-J Day, the 504th dropped supplies to Allied prisoners, participated in show-of-force missions, and flew over Japan to evaluate bombardment damage.