Adrian Weale, from Eye-Witness Hiroshima

Soon after the discovery, in 1939, of fission in uranium by the German scientists Otto Hahn and Fritz Strassman, the American government launched a massive research program at Los Alamos, New Mexico, with a view to developing an atomic bomb. The first of these was tested near Alamogordo, New Mexico, on July 16, 1945. Less than a month later, another bomb - nicknamed "Little Boy" - was loaded into a B-29 bomber, Enola Gay, which on August 6 was one of three aircraft that headed for the Japanese city of Hiroshima.

Adrian Weale's account of what followed is based on the testimony of the crew, including that of the pilot, Colonel Paul Tibbets; the co-pilot, Captain Robert Lewis; the navigator, Major "Dutch" van Kirk; the bombardier, Major Thomas Ferebee; the weaponeer, Captain "Deke" Parsons; the radar officer, Lieutenant Jacob Beser; and the tail-gunner, Sergeant Bob Caron.

Enola Gay reached its bombing altitude at 7.40 a.m. and ten minutes later achieved landfall to the east of Hiroshima. Now cruising at an equivalent groundspeed of about 325 miles an hour, they made time quickly towards their target. At 8.12 a.m., van Kirk announced that they were at the initial point; at the same time, the aircraft were visually sighted by ground observers based to the east of Hiroshima. As Tibbets steadied the plane for the final approach, a message was being cranked out from a local air defence centre:

8.13 Chugoku Regional Army Reports three large enemy planes spotted, heading west from Saijo. Top Alert.

The final run was the responsibility of Ferebee. According to Tibbets:

Twelve miles from the target, Ferebee called, "I can see it!" He clutched in his bombsight and took control of the plane from me for a visual run. . . . [he] had the drift well killed but the rate was off a little. He made two slight corrections. A loud "blip" on the radio notified the escort B-29s that the bomb would drop in two minutes. After that, Tom looked up from his bombsight and nodded to me: it was going to be okay. He motioned to the radio operator to give the final warning. A continuous tone went out, telling [the escorts]: "In fifteen seconds she goes."

The tone was heard by all the other aircraft on mission, including the weather scouts and Top Secret, the stand-by plane on Iwo Jima. Major McKnight, Top Secret's pilot, told the communication officer, who radioed back to the 509th:

It's about to drop.

At exactly 8.15:17 a.m., the "Little Boy" was released from the bomb bay of the Enola Gay. The plane lurched upwards as the weight of the 9,000 lb bomb ceased to bear on it, but it still seemed to Ferebee as if the bomb was keeping pace with them. He watched through the nose as it began to fall away:

It wobbled a little until it picked up speed, and then it went right on down just like it was supposed to.

As the bomb left, Tibbets needed to get the Enola Gay as far from the bomb as possible:

I threw off the automatic pilot and hauled Enola Gay into the turn. I pulled anti-glare goggles over my eyes. I couldn't see through them; I was blind. I threw them to the floor.

The bomb continued falling, its in-built radar methodically measuring the distance from the ground as it fell towards the T-shaped Aioi bridge, described by Tibbets as "the most perfect aiming point I had seen in the whole war"; its outer casing scrawled with messages from the 509th ground crew, including: "Greetings to the Emperor from the men of the Indianapolis." At 5,000 feet the barometric safety switch operated and, as the "Little Boy" reached 1,900 feet, the proximity fuse fired, sending the U235 bullet down the short barrel of the gun assembly into its U235 target. The super-critical mass was formed, drenched in neutrons by the polonium/beryllium initiator, and an uncontrolled chain reaction went through eighty generations before the expanding uranium core was too large to sustain it.

As Tibbets strained to get Enola Gay away to the south, "A bright light filled the plane." Watching, stunned, from his position in the rear of Enola Gay, Sergeant Bob Caron, the tail gunner, noticed a strange ripple in the air coming towards him. He tried to shout a warning but was too incoherent; the first shock wave hit them. Tibbets was astonished:

We were eleven and a half miles slant range from the atomic explosion but the whole airplane crackled and crinkled from the blast. I yelled "Flak!" thinking a heavy gun battery had found us.

Ferebee shouted:

The sons of bitches are shooting at us!

Caron saw the second shock wave:

There's another one coming!

Van Kirk thought the sensation was:

very much as if you've ever sat on an ash can and had somebody hit it with a baseball bat . . . the plane bounced, it jumped and there was a noise like a piece of sheet metal snapping.

Tibbets realized what was happening:

Okay. That was the reflected shock-wave, bounced back from the ground. There won't be any more. It wasn't Flak. Stay calm.

Tibbets ordered Beser to start recording the crew's impressions of the blast, starting with Caron, the only one looking directly at the bomb when it exploded:

A column of smoke is rising fast. It has a fiery red core. A bubbling mass, purple grey in colour, with that red core. It's all turbulent. Fires are springing up everywhere, like flames shooting out of a huge bed of coals. I am starting to count the fires. One, two, three, four, five, six... fourteen, fifteen... it's impossible. There are too many to count. Here it comes, the mushroom shape that Captain Parsons spoke about. It's coming this way. It's like a mass of bubbling molasses. The mushroom is spreading out. It's maybe a mile or two wide and half a mile high. It's growing up and up and up. It's nearly level with us and climbing. It's very black, but there is a purplish tint to the cloud. The base of the mushroom looks like a heavy undercast that is shot through with flames. The city must be below that. The flames and smoke are billowing out, whirling out into the foothills. The hills are disappearing under the smoke. All I can see now of the city is the main dock and what looks like an airfield. That is still visible. There are planes down there.

Tibbets remembered:

Lewis pounding my shoulder, saying, 'Look at that! Look at that! Look at that!' Tom Ferebee wondered about whether radioactivity would make us all sterile. Lewis said he could taste atomic fission. He said it tasted like lead.

Watching, Lewis cried out:

My God! Look at that son-of-a-bitch go!

But in the log that he was keeping of the mission, he wrote:

My God, what have we done?