This is almost definitely "[Noise Level][1]" (1952) by Raymond F. Jones.

The only difference is that instead of telling the scientists that the Soviets had developed working anti-gravity, the project leader showed the assembled scientists a (faked) movie showing the anti-gravity device in operation, and claimed that the government had lost his notes.

The scientists do ultimately succeed in developing anti-gravity, though they initially think their efforts are a failure because their machine is much bulkier and uses a lot more power than the one they were shown.

  [1]: http://www.isfdb.org/cgi-bin/title.cgi?55972