As Jtsummers linked Wikipedia article explains, the term 'Think Tank' was originally applied to the RAND corporation, which was started by the Air Force in 1947

As Jtsummers linked Wikipedia article explains, the term 'Think Tank' was originally applied to the RAND corporation, which was started by the Air Force in 1947 as an R&D organization within Douglas Aircraft, but soon became an independent company which was still largely funded by contracts from the US Department of Defense. RAND was enormously influential in US and world politics and technology in the 1950s, 60s and 70s. People who worked at RAND included the nuclear strategist Herman Kahn, who wrote a horrifying book On Thermonuclear War and become one of the models for Dr. Strangelove (in that movie it's called the BLAND Corportation), Daniel Ellsberg, who leaked a copy of the Pentagon Papers secret history of the Vietnam War he found at RAND, and Paul Baran, who proposed a national packet switched computer network, which provided the technical foundations for the Arpanet and later the Internet. The book The Wizards of Armageddon by Fred Kaplan tells the story of the early days at RAND. http://www.fredkaplan.info/wizards.htm http://www.fredkaplan.info/wizards.htm Availble tech reports from RAND include some of Herman Kahn's early work: https://www.rand.org/pubs/authors/k/kahn_herman.html https://www.rand.org/pubs/authors/k/kahn_herman.html and Paul Baran's computer networking proposal, as well as some remarkably prescient essays on the social and political implications: https://www.rand.org/pubs/authors/b/baran_paul.html https://www.rand.org/pubs/authors/b/baran_paul.html Reports like these (as well as the secret ones that are never released to the public) are the products of a think tank. Think tanks exist to produce these reports, and to support their authors and other spokespeople who publicize and advocate for them.