h turned out to be correct, the summer of
1945 was named as the most likely date when sufficient production would
have been achieved to make it possible actually to construct and
utilize an atomic bomb. It was essential before this time to develop
the technique of constructing and detonating the bomb and to make an
almost infinite number of scientific and engineering developments and
tests. Between the fall of 1942 and June 1945, the estimated
probabilities of success had risen from about 60% to above 90%;
however, not until July 16, 1945, when the first full-scale test took
place in New Mexico, was it conclusively proven that the theories,
calculations, and engineering were correct and that the bomb would be
successful.

The test in New Mexico was held 6 days after sufficient material had
become available for the first bomb. The Hiroshima bomb was ready
awaiting suitable weather on July 31st, and the Nagasaki bomb was used
as soon after the Hiroshima bomb as it was practicable to operate the
second mission.

The work on the actual selection of targets for the atomic bomb was
begun in the spring of 1945. This was done in close cooperation with
the Commanding General, Army Air Forces, and his Headquarters. A
number of experts in various fields assisted in the study. These
included mathematicians, theoretical physicists, experts on the blast
effects of bombs, weather consultants, and various other specialists.
Some of the important considerations were:

A. The range of the aircraft which would carry the bomb.

B. The desirability of visual bombing in order to insure the most
effective use of the bomb.

C. Probable weather conditions in the target areas.

D. Importance of having one primary and two secondary targets for each
mission, so that if weather conditions prohibited bombing the target
there would be at least two alternates.

E. Selection of targets to produce the greatest military effect on the
Japanese people and thereby most effectively shorten the war.

F. The morale effect upon the enemy.

These led in turn to the following:

A. Since the atomic bomb was expected to produce its greatest amount
of damage by primary blast effect, and next greatest by fires, the
targets should contain a large percentage of closely-built frame
buildings and other construction that would be most susceptible to
damage by blast and fire.

B. The maximum blast effect of the bomb was calculated to extend ov