General Groves was highly impressed with Curtis Lemay, as he explains in Now It Can Be Told: The Story of the Manhattan Project:
It was very evident that he was a man of outstanding ability. Our discussion lasted about an hour, and we parted with everything understood and with complete confidence in each other. This feeling lasted throughout the operation and into the years since then.
I explained to him the anticipated outcome of our work, describing the probable power of the bombs, their expected delivery dates and probable production rates, and said that we fully expected to drop each bomb as soon as it was ready. I also went into the general organization and state of training of the 509th Group; the responsibilities of the supporting groups from Los Alamos; the factors governing the altitude from which the bomb would have to be dropped, which was approximately the maximum altitude of the B-29; the approximate weights of the two types of bomb; the targets that we had selected; and the type of instructions that would be issued to the field. I made it perfectly clear that the conduct of the operation would be entirely under his control, subject, of course, to any limitations that might be placed upon him by his instructions. Finally, I explained the roles of the two weaponeers, Parsons and Ashworth—the men who would actually arm the bomb—giving him a resume of their particular qualifications.
LeMay asked a few very pertinent questions, and then announced that he would want to carry out the bombing operation using a single unescorted plane. In explaining his reasons for preferring this radical tactic, he pointed out that the Japanese were unlikely to pay any serious attention to a single plane flying at a high altitude, and would probably assume that it was on either a reconnaissance or a weather mission. I replied that I thought his plan was sound, but that this phase of the operation came under his responsibility. I added, however, that some arrangement should be made for the necessary observation planes to be present in the general area at the time the bomb was dropped.
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Because they had been modified to carry the atomic bomb, the B-29’ s of the 509th Group could not easily carry standard conventional bombs. They could, however, deliver bombs having the same shape as the Fat Man, and such a bomb had been developed and produced to provide training and experience to the crews. Known as the Pumpkin, this bomb contained 5,500 pounds of explosives, and was designed for blast effect only, with a proximity fuse that would permit its use for an air burst. Although it was primarily a training device, we had always recognized that it could have tactical uses; now as part of the group’s security cover, we let it leak out on Tinian that its mission was the delivery of Pumpkins in battle. We also hoped that analysis of the results obtained by the use of the Pumpkins might help us to refine the ballistic data for the real bomb.
The Pumpkins began to arrive at the end of June. Reaction to these bombs were mixed. The members of the 509th who, with a few exceptions, still did not know the real reason for their training, were somewhat disappointed that they had spent so much time in practicing to deliver this fairly modest weapon. On the other hand, some members of the other Air Force units based on Tinian, who likewise did not know what the 509th’s real purpose was, became quite enthusiastic about the effectiveness of the Pumpkin’s air bursts over enemy targets and set up a clamor to have more of them made available to their theater.
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As I have explained, a high air burst was necessary for maximum results. It was also dictated by our desire to eliminate, if possible, or in any case to decrease, residual radioactivity on the ground below the burst; to decrease to a negligible degree any harmful fallout downwind; and to diminish to a minimum serious radioactive injuries to the population in the bombed area. We felt that the high burst would confine casualties for the most part to nonradioactive injuries; namely, those due directly and indirectly to the force of the unprecedented explosion.
To be well removed from the point of burst, the bombing plane would have to maneuver as no heavy bomber had ever had to maneuver before. As soon as the bomb was “away,” the plane was to make a sharp diving turn to get as far as possible from the point of explosion. This was one of the reasons why the run was made at the then unprecedented altitude of some thirty thousand feet. The high altitude also greatly reduced the danger of gunfire from enemy airplanes, permitting the removal of the fuselage turrets and all other armament except for the tail guns. This weight reduction appreciably increased the plane’s range and the height at which it could fly.
Studies made at Los Alamos had determined that with a bomb of twenty thousand tons of TNT equivalent, a B-29 plane ten miles away from the burst would be safe from destruction by a factor of two. Under these conditions, the aircraft, which had been designed to withstand a force of four times gravity, would be subjected to a force equivalent to no more than two times gravity. It was calculated that by making a sharp diving turn, the sharpest possible consistent with safety, the B-29 could reach a point at least ten miles from the burst by the time the bomb exploded.