The 393rd Bombardment Squadron crew of the B-29 Enola Gay. 

During the final stages of World War II in 1945, the United States conducted two
atomic bombings against the cities of Hiroshima and Nagasaki in Japan, the first
on August 6, 1945 and the second on August 9, 1945. For six months prior to the bombings, the United States fire-bombed 67 Japanese cities and  calling  for the surrender of Japan in a declaration issued on July 26, 1945. The Japanese
government ignored the ultimatum and by executive order of President Harry S.
Truman, the U.S. dropped the nuclear weapon “Little Boy” (shown below) on the
city of Hiroshima on Monday, August 6, 1945, followed by the detonation of the
second atomic bomb, “Fat Man” over Nagasaki on August 9. Within the first two
to four months of the bombings, the acute effects killed 90,000–166,000 people
in Hiroshima and 60,000–80,000 and 60,000–80,000 in Nagasaki,with roughly
half of the deaths in each city occurring on the first day. The Enola Gay (above)
dropped “Little Boy” on Hiroshima.

The atomic bomb “Little Boy”

 
The explosion of “Little Boy”

Hiroshima after the bombing