The “Little Boy” bomb dropped on Hiroshima was armed in midair rather than before takeoff due to safety concerns. This was long thought to be the idea of ordnance expert Deak Parsons, who armed the bomb aboard the Enola Gay. This memo from Parsons’s associate Norman Ramsey, however, reveals that the Weapons Committee had safety concerns long before and recommended that the bomb be armed after takeoff.
9 July, 1945
From: N. F. Ramsey
Subject: Dangers from Accidental Detonations of Active Gadgets
1. The seriousness of accidental detonations of active gadgets has been discussed at some length by the Weapons Committee. Consistent with the tight time schedules and newness of our device every effort has been made to reduce the chances of such an accidental detonation to the lowest possible value. Nevertheless, the chances of such a catastrophe although small are nevertheless finite. Such a catastrophe would be very serious if a large nuclear explosion resulted, in that it could neutralize a considerable fraction of an important Army advance base. For this reason it is the view of the members of the Weapons Committee that the existence of such a hazard should be called to the attention of General Groves in order that it is certain that the decision to use the gadget in its present form is made with the knowledge of the existence of these risks.
2. It was the unanimous feeling of the weapons Committee that one of the more serious dangers for a high order nuclear explosion was the firing of the gun gadget by a possible fire caused by crashing in take off. The Weapons Committee therefore agreed with [Francis] Birch’s recommendation that the gun be loaded in flight subsequent to take off. Birch assured the committee that this operation can be sufficiently simple that the gain in safety more than compensates for the small loss in reliability.
[Thanks to 509th Composite Group historian Joseph Papalia for bringing this document to AHF's attention.]