In a telephone interview, Mr. Billings recalled how unflappable Mr. Mayer was on the plane as he readied his parachute for the jump and gave a few final directives.

Image Mr. Mayer in 2013. Credit... Office of Sen. Jay Rockefeller

“I was in awe of him,” Mr. Billings said. “He was born without the fear gene. He feared nothing, and he was able to be whatever he needed to be.”

Mr. Mayer was born on Oct. 28, 1921, in Freiburg, Germany, on the edge of the Black Forest. Known as a boy as Fritz, he had his bar mitzvah in Germany just as Hitler and the Nazis were rising to power in 1933, but he never considered himself terribly religious.

With the Nazis’ systematic anti-Semitism growing more onerous in the mid-1930s, Fritz and his mother pushed his father, who had a metal-fabricating shop, to flee the country. But his father resisted; he had served in the German Army in World War I, receiving an Iron Cross, and he told his son that the Nazis would never go after a war veteran, even one who was Jewish.

The elder Mr. Mayer finally relented in 1938, months before the start of the mass deportations and ultimately the genocide of the European Jews, and the family made its way to Brooklyn when the younger Mr. Mayer was 16. He insisted on being called Fred in his new country; he never answered to Fritz again, saying it reminded him of Nazi Germany.

Trained as a mechanic, he enlisted in the Army on Dec. 8, 1941, a day after the bombing of Pearl Harbor. Early assignments at bases in Arizona, Georgia and Maryland bored him, he said, and when he had the chance to train for covert missions in Italy as a corporal with the O.S.S., he quickly put his hand up.