I'm pretty sure this is "Noise Level" by Raymond F. Jones.
http://www.sf-encyclopedia.com/entry/jones_raymond_f:
"Noise Level" (December 1952 Astounding), an archetypal Astounding tale of Conceptual Breakthrough, scientific advance taking place through destruction of a previous paradigm: Scientists are told that Antigravity exists, and so proceed to invent it.
"Noise Level" on ISFDB
