Like many students his age, he had been forced by Japan's government to support the wartime effort only to find himself a victim of the world's first nuclear attack, on Hiroshima.
The office building was three kilometers from the epicenter of the bomb that the United States released to end World War II. It collapsed in the blast, trapping Katsumi in the rubble.
As it turned out, that rubble protected him from being seared by the intense thermal heat that killed so many people. Some 140,000 people died in the attack, with 70,000 more deaths resulting from the bombing of the port city of Nagasaki three days later.
Digging himself out, he proceeded to rescue many people from the ashes, exposing himself to massive amounts of radiation that decades later would kill him.
But he lived to recount the harrowing experience to his son, Atsushi Okudera.
Atsushi is now a journalist based in Washington, writing about national security as part of the State Department press corps for Asahi Shimbun, a leading Japanese newspaper.
With his father's experience foremost in his mind and heavy on his heart, Atsushi traveled to Hiroshima to cover the landmark Group of Seven meetings this week that paid tribute to the city's devastating past in the hopes the visit would instill upon leaders the importance of a world without nuclear weapons.
It came as GOP presidential front-runner Donald Trump has raised the possibility that Japan arm itself with nuclear weapons to deter a threat from North Korea, sending chills down the spines of many who experienced the devastation of a nuclear bomb first-hand.
A group of journalists, Atsushi included, on Monday visited the Hiroshima Memorial Peace Museum documenting the devastating impact of the attack, including victims' and survivors' burnt clothing and personal effects.
To visit this powerful memorial is to understand the devastating experience that gave birth to a pacifist tradition in Japan that continues to this day.
When the group, which included Secretary of State John Kerry, came across the charred tricycle of a 3-year-old boy who perished near ground zero, Atsushi grew quiet.
"It's been more than 20 years since I've been to this museum. I forgot the importance of what happened in here," he said. "I'm ashamed."
Katsumi survived Hiroshima to become a technician at a leading Japanese manufacturing company and played baseball with his son after a long hard day at the factory.
Atsushi said his father didn't talk often about that fateful day when the world changed. But his death at 56 of prostrate cancer, a common affliction among those exposed to the radiation from the blast, hangs over his family.
He embodies the collective consciousness of a nation whose constitution makes a solemn oath never to use nuclear weapons and to avoid war at all costs.
"The Japanese people have this experience of Hiroshima. They know the consequences of war," Atsushi said. "Many were very poor and suffered a great sacrifice because of the actions of a wartime government."
He paused for a few seconds and added: "People here really hate war."
That's why Trump's suggestion that Japan and South Korea might need to step out from under the U.S. security umbrella and develop their own nuclear arsenal cut so deep.
"At some point, we have to say, you know what, we're better off if Japan protects itself against this maniac in North Korea," Trump said at a CNN town hall in March. "We're better off, frankly, if South Korea is going to start to protect itself."
On Sunday at a rally in Rochester, New York, he spoke of the need for Japan to pay more for the defense the United States provides.
"I want to treat them fairly," he said. "With Japan, I'd say very nicely, 'Listen we need help, you got to take care, we don't want you to arm, but you know what, you have to be prepared.' "
"The Japanese people are angry. They really hate this kind of selfish opinion," Atsushi said Monday. "Donald Trump doesn't know what happened in Hiroshima or how miserable the situation was in 1945. He has no imagination about what would happen if Japan possessed a nuclear weapon."
President Barack Obama also expressed outrage at the idea, questioning Trump's fitness to be commander in chief.
The comments, the President said, show Trump "doesn't know much about foreign policy or nuclear policy or the Korean Peninsula or the world generally."
Referring to the use of nuclear weapons as one of the gravest responsibilities of an American president, Obama said, "We don't want somebody in the Oval Office who doesn't recognize how important that is."
The President lamented that Trump's "wacky ideas" were a diversion in his private conversations with world leaders at last month's Nuclear Security Summit, where 50 world leaders gathered in Washington to discuss ways to reduce the threat of a nuclear attack and were equally bent on having Obama or his top diplomat explain Trump's foreign policy rationale and his appeal to Republican primary voters.
The Trump campaign did not respond to a request for comment.
It is John Kerry, who spends a fair share of his time traveling to world capitals, who faces the most intense barrage of questions from leaders and foreign ministers about Trump's provocative foreign policy prescriptions. Aides to the secretary of state got an earful last month when Trump called the NATO alliance "obsolete."
Since then, Trump has declined to rule out using nuclear weapons in a military conflict in Europe. Kerry's foreign minister counterparts, he has said, are "shocked" by some of the rhetoric on national security issues espoused by Trump.
But in no place is the discomfort around Trump's bold foreign policy suggestions felt more acutely than in Japan, the only country to have had nuclear weapons used against it.
The nation's security alliance with the United States has been the bedrock of its foreign policy for decades and, to this day, is viewed as the stabilizing factor in Asia amid an erratic North Korea and an increasingly assertive China.
The mere suggestion the security arrangement be reconsidered has Japan and other U.S. allies in Asia unsettled about the prospect of a Trump presidency.
"I don't think people necessarily have really processed it, in part because it strikes so many Asians as wildly contrary to the understanding of U.S. interests that have been embraced and acted on by administration after administration," a senior aide traveling with Kerry said about Trump's comments.
Kerry himself was far less diplomatic while visiting Hiroshima for G7 meetings.
Along with the group's other foreign ministers, Kerry laid a wreath at a peace memorial footsteps from ground zero and toured the museum, calling it a "gut-wrenching" experience he will never forget.
"It reminds everyone of the extraordinary choices of war and what war does," he told reporters at a news conference.
Then, unprompted, he added: "Any suggestion by any candidate for high public office that we should be building more weapons and giving them to a country like Korea or Japan are absurd on their face and run counter to everything that every president, Republican or Democrat alike, has tried to achieve ever since World War II."
Second-generation Hiroshima survivors such as Atsushi said Trump's comments reflect not only a lack of appreciation for what happened here but also a lack of understanding of the arms race that would ensue in Asia if Japan were to go nuclear.
"People forget about the sacrifice of Hiroshima," he said. "Everybody needs to come here and see what happened here on August 6, 1945, how real it is when you make a decision to use a nuclear weapon. If we start to use nuclear weapons today, that is the end of the world."