I think it is incorrect
I think it is incorrect. I am also trying to find out actual reason.
"The normal radio broadcast warning was given to the people that it might be advisable to go to air-raid shelters if B-29s were actually sighted. However a reconnaissance mission was assumed because at 07:31 the first B29 to fly over Hiroshima at 32,000 feet (9,800 m) had been the weather observation aircraft
Straight Flush that sent a Morse code message to the
Enola Gay indicating that the weather was good over the primary target. Because it then turned out to sea, the 'all clear' was sounded in the city. At 08:09 Colonel Tibbets started his bomb run and handed control over to his bombardier.
[61]
The release at 08:15 (Hiroshima time) went as planned, and the
gravity bomb known as "Little Boy", a
gun-type fission weapon with 60 kilograms (130 lb) of
uranium-235, took 43 seconds to fall from the aircraft flying at 31,060 feet (9,470 m) to the predetermined detonation height about 1,900 feet (580 m) above the city. The
Enola Gay traveled 11.5 miles (18.5 km) before it felt the shock waves from the blast."
[62]
Source :
http://en.wikipedia.org/wiki/Atomic_bombings_of_Hiroshima_and_Nagasaki
http://en.wikipedia.org/wiki/Atomic_bombings_of_Hiroshima_and_Nagasaki#cite_note-61