William Leonard Langer (March 16, 1896 – December 26, 1977) was an American historian, intelligence analyst and policy advisor.

He was on leave during World War II as head of the Research and Analysis Branch of the Office of Strategic Services.

Born in South Boston, Massachusetts on March 16, 1896, he was the second of three sons of recent German immigrants, Charles Rudolph and Johanna Rockenbach.

He taught modern European history at Clark University for four years before accepting an assistant professorship at Harvard.

In correspondence he was identified as OSS 117,[14] a codename which entered French popular culture in 1949 for an unrelated iconic fictional character of books and film.

In 1950 Langer organized the office of National Estimates in the newly established Central Intelligence Agency.

[15][non-primary source needed] After the war, Langer returned to academia, but from 1961 to 1977 he served on the President's Foreign Intelligence Advisory Board.

[16] Langer’s book was then used as new evidence to request a review of the trial of Pétain by his lawyer in 1950 (the case was eventually dismissed).

[18] William Langer was awarded the Medal for Merit by President Truman in July 1946 in recognition of his wartime service.