Goofs

The B-29 shown as the Enola Gay taxiing before takeoff has, incorrectly, its defensive armament of machine guns. The B-29 as the Enola Gay shown taking off and flying the rest of the mission correctly does not.

The view from the cockpit of the Enola Gay about to take off shows the plane "The Great Artiste" in the background, even though it had already taken off just moments before.

In the movie the character Matt Cochran (played by Tom Drake) has an accident in the laboratory on Tinian that eventually kills him from radiation poison, but he is credited with saving 40,000 lives because of his self-sacrifice of bare-handedly separating the radioactive materials. This incident did not happen on Tinian. Rather, it reflects a similar accident that killed Canadian scientist Louis Slotin at Los Alamos NM in May 1946.

The original Trinity Site bomb (called "the gadget" by the scientists) detonated in New Mexico in July 1945, was not the sleek, traditional bomb shape shown in the movie. The gadget was spherical and covered externally with wires.

The planes on the Hiroshima mission did not encounter any anti-aircraft fire as depicted in the film.

The three bombers of the Hiroshima strike group are shown taking off in broad daylight when they actually took off in the middle of the night - around 03:00 local time.

The "Little Boy" bomb that was dropped on Hiroshima was not aerodynamically-shaped like the one in the movie. Instead, it was cylindrical with a rounded blunt end and a box-like fins. Additionally, it was not loaded in Enola Gay as depicted in the movie; the bomb was placed in a pit and the plane was moved over it for uploading.

In all the close-ups of the B-29's where the crew is shown looking out, there is no glass in any of the windows.

Right before the first bomb test in the desert, one of the last preparation steps on the tower supporting the bomb purportedly was to use a screwdriver and hammer to engage 3 locations hidden inside the bomb.

The actor did a poor job of simulating the task regardless of whether it was an actual step or not, since the hand holding the screwdriver was moving unsteadily rather than fixed against the mechanisms to be engaged.

Air traffic at National Airport shows aircraft at an airport in a desert with mountains in the background. But National Airport is next to the Potomac with many buildings in close proximity.

At the beginning of the movie, the character J. Robert Oppenheimer addresses the audience from his desk in a science laboratory. In reality, Oppenheimer was a theoretical physicist, and by his own admission was quite clumsy in a physics lab.

Einstein at one point calls Rutherford "an Englishman". He was actually from New Zealand, although that was a British colony at the time of his birth.