We're going back in time a little for the next hour or so. I am going to live-tweet the atomic bombing of Hiroshima, which occurred on 6 August 1945. Exactly 75 years ago.

This is a sad, horrific day but we need to remember it.

T-1 hour. The bombers are en route, having taken off from Tinian. B-29 Superfortresses, the plane that carries the atomic device - Little Boy - is the Enola Gay. The name was after the pilot, Col. Paul Tibbets.

She was only named on 5 August.

*Correction: The Enola Gay was named after Tibbet's MOTHER. She was taken from the usual pilot for the mission.

They set off about 4 1/2 hours ago. The Enola Gay's callsign is Dimples 82. There are 6 other planes: three for weather recon over targets, one strike obs, one spare.

Enola Gay picks up a message from Dimples 85: "Straight Flush". The plane's job is to assess cloud over Hiroshima.

"Cloud cover less than 3/10th at all altitudes. Advice: bomb primary."

Hiroshima will be the target.

Down below, the city has just sounded the all-clear.

The mission includes one future Nobel prize winner. Accompanying Enola Gay is The Great Artiste, which will do measurements of the blast. On board is Luis Alvarez, who will win the Physics prize in 1968 for developing the hydrogen bubble chamber.

The bomb is a uranium atomic weapon, a type that has never been detonated before: the first test was a plutonium, more powerful, 'Fat Man' bomb. Little Boy is a misnomer, though: the bomb weighs 9700 pounds and is around 3m long.

It is already armed.

Tibbets has ordered no formation flying. The planes with him - Great Artiste and (I am not making this name up) Necessary Evil, are just in visual range.

The Sun is up now. They are slowly rising to about 31,000 feet. On board, the crew says very little.

In Hiroshima, people are beginning their day. They do not know about atomic weapons. It is HQ of General Hata's 2nd Army, controlling the defence of southern Japan. Around 350,000 people live here.

The target of the bomb is the strange, T-shaped Aioi Bridge in the city heart.

T-30 minutes. On board Enola Gay, the mission commander, William "Deak" Parsons, does the final arming, removing the safety devices that could have caused it to detonate if it crashed on take-off. The duty falls to his assistant, Morris Jeppson.

There are 12 men on board. Only one of them, Robert Lewis, is keeping a minute-by-minute account.

Japan comes into view. For Navigator "Dutch" Van Kirk, it is the first and only mission he will fly over the country.

The Enola Gay's flightpath goes over the island of Shikoku and on to Hiroshima. The city becomes clear into view about 50 miles out.

It's a beautiful day.

A total of 3,243 troops have begun physical training in the ground of Hiroshima Castle.

They will all be dead in 15 minutes.

T-13 minutes. Bombardier Thomas Ferebee, in the nose, recognises Hiroshima. He asks Dutch, who confirms they have the target. They will make their bomb run shortly.

T-6 minutes. Ferebee takes control of the plane. The Enola Gay has begun its bombing run. Only Ferebee, Tibbets and Parsons know what the bomb can do: everyone else has been given black goggles and told to expect a flash.

This is what Hiroshima looks like from the air.

Eizō Nomura, is working in a concrete basement of the Rest House, about 200m from where the blast will happen. He will be the closest survivor of the atomic bomb attack.

The other 36 people in the building above him will die.

T-2 minutes. Tibbets begins to count down to his crew.

The worst weapon in history will soon be used.

"10...9...8...7..."

8.15am 6 August 1945. Enola Gay releases the atomic bomb.

It will take 43 seconds to reach the ground. It is already caught in a crosswind and will miss the target. Enola Gay veers upward and immediately begins a 160-degree turn to escape.

The bomb misses its target, and detonates directly over the Shima Surgical Clinic. It releases 16 kilotons of energy, levelling everything in a mile’s radius of the blast. There is a flash… then a blast. Then death.

Within a four-mile radius, fires rage.

Devestation. Carnage. Thousands die instantly. Thousands more will die soon. Around 70% of Hiroshima’s buildings are destroyed. Among the casualties is the mayor, killed instantly while eating breakfast with his son and granddaughter.

Enola Gay is buffeted by the explosion, the men in the flight cabin dazzled like ‘a photographer’s bulb going off’. They continue to fly away, in an arc northeast then southeast of the city. From their vantage point, they see only dust and smoke, “like a pot black, boiling tar.”

On the ground, the death toll is catastrophic. Many more are severely injured, such as from thermal flash burns. Already a firestorm has begun to sweep through Hiroshima. There is no one to help: 90% of the city’s doctors and nurses are casualties themselves.

At the Red Cross Hospital, only one doctor, Terufumi Sasaki, is able to remain on duty and help survivors.

Yoshie Oka, a high school student, messages Fukuyama HQ: “Hiroshima has been attacked by a new type of bomb. The city is in a state of near-total destruction."

Field Marshal Hata is only slightly wounded and trying to take control but many of his staff are dead.

12 American airmen are imprisoned in the city. Eight die instantly; two are executed; two are badly wounded and left to be stoned to death next to Aioi Bridge.

The mushroom cloud has begun to form over the city. On the outskirts, fires rage. Tibbets, exhausted, hands over controls and decides to take a nap. Enola Gay returns safely to base and will arrive at 3pm.

In the carnage, a firestorm has begun (pictured).

In total, 20,000 Japanese soldiers and 70-120,000 civilians will lose their life. It is the worst death toll caused by a single weapon in history.

Three days later, a more powerful bomb will be dropped on Nagasaki.

Please remember why we do not use these weapons. #Hiroshima75

This has been a very horrible thread to write but it is important we remember.

This story needs to be told, shared, and memorialised.

Lest we forget. #Hiroshima75

For those wondering, Enola Gay is currently a museum display at the Udvar-Hazy Center near Dulles Airport in Virginia, US (part of the Smithsonian).