By Matthew Davis
BBC News, Washington
The crew of the Enola Gay posed for photographs before their mission
They were young men hoping to help end World War II. But to their mission's critics, the crews that dropped the atomic bombs on Japan were part of a war crime.
Three men involved in the attack on Hiroshima told the BBC about their memories of a day that has stayed with them for 60 years.
Theodore "Dutch" Van Kirk, 84
The day before the mission we sat through briefings on Tinian island where they told us who was assigned to which plane, and we ran through what we were going to do.
Mr Van Kirk (left) has never doubted the bomb's morality
About 2pm we were told to get some sleep. But I don't know how they expected to tell us we were dropping the first atomic bomb on Japan and then expect us to sleep.
I didn't get a wink. Nor did most of the others. But at 10pm we had to get up again because we were flying at 2.45am.
They briefed us that the weather was good, but they were sending weather observation planes up so we would have the best information on targeting Hiroshima.
We had a final breakfast and then went down to the plane shortly after midnight.
There was a lot of picture-taking and interviewing going on - by the military - and it was a relief to get in the Enola Gay about an hour before we took off.
We flew in low over Iwo Jima while the bomb crew checked and armed Little Boy (the uranium bomb) and once we cleared the island we began climbing to our bombing altitude of just over 30,000 feet.
It was perfectly clear and I was just doing all the things I'd always done as a navigator - plotting our course, getting fixes to make sure we were on course and reading the drifts so we knew the wind speed.
As we flew over an inland sea I could make out the city of Hiroshima from miles away - my first thought was 'That's the target, now let's bomb the damn thing'.
But it was quiet in the sky. I'd flown 58 missions over Europe and Africa - and I said to one of the boys that if we'd sat in the sky for so long over there we'd have been blown out of the air.
Once we verified the target, I went in the back and just sat down. The next thing I felt was 9,400lbs of bomb leaving the aircraft - there was a huge surge and we immediately banked into a right hand turn and lost about 2,000 feet.
We'd been told that if we were eight miles away when the thing went off, we'd probably be ok - so we wanted to put as much distance as possible between us and the blast.
All of us - except the pilot - were wearing dark goggles, but we still saw a flash - a bit like a camera bulb going off in the plane.
There was a great jolt on the aircraft and we were thrown off the floor. Someone called out 'flak' but of course it was the shockwave from the bomb.
Within a minute of the blast a white cloud had reached 42,000ft
The tail-gunner later said he saw it coming towards us - a bit like the haze you see over a car park on a hot day, but moving forwards at great speed.
We turned to look back at Hiroshima and already there was a huge white cloud reaching up more than 42,000 feet. At the base you could see nothing but thick black dust and debris - it looked like a pot of hot oil down there.
We were pleased that the bomb had exploded as planned and later we got to talking about what it meant for the war.
We concluded that it would be over - that not even the most obstinate, uncaring leaders could refuse to surrender after this.
In the weeks afterwards, I actually flew back to Japan with some US scientists and some Japanese from their atomic programme.
We flew low over Hiroshima but could not land anywhere and eventually landed at Nagasaki.
We didn't hide the fact that we were American and many people turned their faces away from us. But where we stayed we were made very welcome and I think people were glad that the war had ended.
Morris "Dick" Jepson, 83
I was a young second lieutenant in the US Air Force and was designated as the weapons test officer on the Enola Gay.
For Dick Jepson, the Enola Gay flight was his first combat mission
The bomb was designed to detonate when it was about 1,500 feet - or about one-and-a-half seconds - above the ground to ensure the maximum possible destructive range.
To that end it contained a range of radar-designated electronics.
In the run-up to the mission I had spent five months at Harvard and three months at MIT studying radar design.
For several months I worked on developing the electronics that would allow the bomb to detonate above the ground, flying test missions over southern California.
The Manhattan Project [to build the atomic bomb] was compartmentalised so the thousands of people working on it could not know the full details of the plan, but I was in no doubt I was training for an atomic bomb drop.
On the day of the mission, I had to perform some final tests on the electronics that operated the bomb.
There was a box in the plane's forward compartment that connected to the bomb via a cable system.
My final job was to climb down into the bomb bay, crawl around the bomb and manually arm the device. I took out three testing plugs that isolated the bomb and put in three red firing plugs.
The most important thought in my mind was that this would detonate and end the war.
Unlike the others, this was the only combat mission I had been on, but there was only one point when I was apprehensive.
I knew how long it took for the bomb to fall and detonate - 43 seconds - so I counted but nothing happened. I just thought this was devastating.
But in the excitement I had counted too fast. That second, the crew reported a huge flash and it had gone off.
A few seconds later I felt the first blast wave.
There was a second shockwave and I knew by the delay that it had detonated at the right height - and this second wave was the force of the bomb bouncing back off the ground.
Everyone's thoughts turned to what devastation there would have been down below - we all had that thought on our mind because we had seen what the bomb could do.
But it was the right thing to do.
Dr Harold Agnew, 85
I had come from working on the Manhattan Project at Los Alamos and my abiding memory is of it being a very exciting time, working with all the best scientific minds of the day.
Dr Harold Agnew, with the plutonium core of the Nagasaki bomb
I describe myself as a 'grunt' at that time, I did what I was told to do. But I was part of a great undertaking.
For the Hiroshima mission I was on board The Great Artiste, a second B-29 that had tailed the Enola Gay to the bombing zone.
We'd flown alongside them all the way up there and were about four or five miles off to one side of Hiroshima, dropping gauges with parachutes that would measure the yield of the bomb.
After we dropped our gauges I remember we made a sharp turn to the right so that we would not get caught in the blast - but we still got badly shaken up by it.
I don't think anyone realised exactly what would happen. It was the only uranium bomb to be dropped. [The Nagasaki bomb used plutonium].
My honest feeling at the time was that they deserved it, and as far as I am concerned that is still how I feel today.
People never look back to what led up to it - Pearl Harbour, Nanking - and there are no innocent civilians in war, everyone is doing something, contributing to the war effort, building bombs.
What we did saved a lot of lives in the long run and I am proud to have been part of it.
After the war I returned to the University of Chicago to continue my studies and later rejoined Los Alamos, where I eventually became director of the laboratory.
About three-quarters of the US nuclear arsenal was designed under my tutelage at Los Alamos. That is my legacy.