Lt George Bush, then a 20-year-old pilot, was among nine airmen who escaped from their planes after being shot down during bombing raids on Chichi Jima, a tiny island 700 miles south of Tokyo, in September 1944 - and was the only one to evade capture by the Japanese.
The horrific fate of the other eight "flyboys" was established in subsequent war crimes trials on the island of Guam, but details were sealed in top secret files in Washington to spare their families distress.
Mr Bradley has established that they were tortured, beaten and then executed, either by beheading with swords or by multiple stab-wounds from bayonets and sharpened bamboo stakes. Four were then butchered by the island garrison's surgeons and their livers and meat from their thighs eaten by senior Japanese officers.
The future president escaped a similar fate because he ditched his plane further from the island than the other crews, and managed to scramble on to a liferaft. American planes launched a hail of fire at Japanese boats which set out to capture him, driving them back, and he was eventually rescued by a US submarine.
When the black hull of the USS Finback surfaced in front of him, he thought he was hallucinating, he told Mr Bradley in a television film made to coincide with the publication of Flyboys. He had been vomiting, bleeding from a head wound, and weeping with fear. He said only four words to his rescuers: "Happy to be aboard."
Mr Bush's part in the raid - for which he won the Distinguished Flying Cross - has long been known to Americans. Not known until now was the grim fate of his downed comrades - none from his own plane - who swam ashore.
Mr Bradley pieced together the horrific truth from secret transcripts of the war crimes trials, given to him by a former officer and lawyer who was an official witness at the time, and the testimony of surviving Japanese veterans.
A radio operator, Marve Mershon, was marched to a freshly dug grave, blindfolded, and made to kneel for beheading by sword, testified a Japanese soldier, named as Iwakawa, at the war crimes trial. "When the flyer was struck, he did not cry out, but made a slight groan."
The next day a Japanese officer, Major Sueo Matoba, decided to include American flesh in a sake-fuelled feast he laid on for officers including the commander-in-chief on the island, Gen Yoshio Tachibana. Both men were later tried and executed for war crimes.
A Japanese medical orderly who helped the surgeon prepare the ingredients said: "Dr Teraki cut open the chest and took out the liver. I removed a piece of flesh from the flyer's thigh, weighing about six pounds and measuring four inches wide, about a foot long."
Another crewman, Floyd Hall, met a similar fate. Adml Kinizo Mori, the senior naval officer on Chichi Jima, told the court that Major Matoba brought "a delicacy" to a party at his quarters - a specially prepared dish of Floyd Hall's liver.
According to Adml Mori, Matoba told him: "I had it pierced with bamboo sticks and cooked with soy sauce and vegetables." They ate it in "very small pieces", believing it "good medicine for the stomach", the admiral recalled.
A third victim of cannibalism, Jimmy Dye, had been put to work as a translator when, several weeks later, Capt Shizuo Yoshii - who was later tried and executed - called for his liver to be served at a party for fellow officers. Parts of a fourth airman, Warren Earl Vaughn, were also eaten and the remaining four were executed, one by being clubbed to death.