On Aug. 6, 1945, the United States dropped an atomic bomb on the Japanese city of Hiroshima. It was the first time a nuclear weapon had been used in warfare.
There were three strike planes that flew over Hiroshima that day: the Enola Gay, which carried the bomb, and two observation planes, the Great Artiste and the Necessary Evil.
Russell Gackenbach was a second lieutenant in the U.S. Army Air Corps and a navigator on the mission. Today, the 95-year-old is the only surviving crew member of those three planes.
Gackenbach enlisted in the Army Aviation Cadet Program in 1943. After completing his training, he was approached by Col. Paul Tibbets, who was recruiting officers for a special mission. Tibbets said it would be dangerous but if they were successful, it could end the war.
The 509th Composite Group, lead by Tibbets, spent months training in Wendover, Utah, before being shipped off to an American air base on the Pacific island of Tinian.
Their planes were reconfigured B-29 Superfortress bombers. They had different engines, fewer guns and a larger bomb bay.
The Enola Gay carried the weapon, nicknamed "Little Boy." It weighed nearly 10,000 pounds and could produce an explosive force equal to an estimated 15,000 to 20,000 tons of TNT.
But at the time, Gackenbach didn't know any of this.
"I never heard the words 'atomic bomb,' " he tells Radio Diaries. "We were only told what we needed to know, and keep your mouth shut."
The planes took off around 2 a.m. on Aug. 6, 1945. Gackenbach was part of the 10-man crew that flew on the Necessary Evil.
"We were told that once the explosion occurred, we should not look directly at it, that we should not go through the cloud," he says. "We were not told anything about the cloud, just [told] don't go through it."
As they made their final approach to Hiroshima, they were flying 30,000 feet over the city. Then, the radio went dead: that was the signal from the Enola Gay that the bomb had been released.
The first thing Gackenbach saw was a blinding light and then the start of a mushroom cloud. He got out of his seat, quickly picked up his camera and took two photographs out the navigator's side window.
The plane circled twice around the mushroom cloud and then turned to head home.
"Things were very, very quiet," Gackenbach says. "We just looked at each other; we didn't talk. We were all dumbfounded."
The casualties on the ground were staggering. An estimated 80,000 people were killed instantly. Another 80,000 died from effects of the bomb in the months and years following. Hiroshima was destroyed.
Three days later, the U.S. dropped a second atomic bomb, on the city of Nagasaki. And on Aug. 15, Japan announced its surrender, bringing an end to World War II.
Gackenbach was discharged in 1947 and went on to work as a materials engineer for 35 years. In 2011, he returned to Japan to visit the Hiroshima Peace Memorial.
"After 73 years, I do not regret what we did that day. All war's hell," he said. "The Japanese started the war; it was our turn to finish it."
This story was produced by Nellie Gilles of Radio Diaries along with Joe Richman and Sarah Kate Kramer and edited by Deborah George and Ben Shapiro. Special thanks to the Atomic Heritage Foundation. This story is part of an ongoing series from Radio Diaries and NPR called Last Witness, which features portraits of the last surviving witnesses to major historical events. Send us your ideas for the series by using the hashtag #LastWitness. To hear more stories from Radio Diaries, subscribe to the podcast at www.radiodiaries.org.