Development of the Mark II,
a brief chronology

1943, 21 August: In report to Vice President Wallace, Secretary of War Stimson and Chief of Staff General Marshall the Atomic
Bomb Military Policy Committee accurately forecast the fair chance that the first atomic bomb, the (uranium) hydride bomb, would be available in the fall of 1944.

1943, 15 September: James Conant requests Rear Admiral William R. Purnell of the Atomic Bomb Military Policy Committee to complete the transfer of 236 pounds (107 kg) enriched uranium hexafluoride from Philip H. Abelson’s Naval Research Laboratory liquid thermal diffusion uranium isotope facility to the Manhattan Project, via General Leslie R. Groves, Military Policy Committee executive officer.

1944, 4 July: James Conant informed General Groves, Atomic Bomb Military Policy Committee, and the Top Policy Committee by the memorandum, “Findings of Trip to L. A. [Los Alamos] July 4, 1944,” that the Mark II was certain enough to be used by the Joint Chiefs of Staff for the purposes of operational planning, but the Mark II would necessarily be proof fired once before the design could be ready for use against the enemy.