Congress was deliberately not informed about the Manhattan Project from the beginning. The fear was, as Roosevelt's advisors suggested, that there would be no way in which a Congressional committee could be convinced of the value of something as costly and risky and sci-fi as the atomic bomb.

However, individual Congressmen did stumble across aspects of the project as the work went on. Harry Truman famously did as a Senator, when he was asked to look into the spending at the Hanford project. There were also Congressmen in Tennessee who got alerts from their constituents that there were huge, secretive works down there (Oak Ridge). In general, as the spending got big, and the amount of labor dedicated to it increased, it became more and more impossible to keep the thing totally under wraps. The Secretary of War, Henry Stimson, and Roosevelt himself at times, worked to quash each of these attempts at an "audit." There were many of them, throughout the project — in 1944 alone there were maybe 6 attempts by Congressmen to learn about the project. Each had to be handled delicately, because there is only so much control the President can exert over individual Congressmen. One of them even threatened to reveal the existence of the project on the floor of open proceedings.

For this reason, in February 1944, Gen. Leslie Groves, Stimson, and Vannevar Bush agreed that it would be necessary to inform a few key Congressmen about the basics of the project, in order to use them to control their other members and force through any budget requests without discussion. They gave very sketchy briefings to the Speaker of the House, the Majority and Minority Leaders of the Senate and the House, and the Senate Chairman and Senior Minority Member of the Senate Appropriations Committee. So seven total Congressmen were told about the work, and only once it was very late in its development. They were not consulted about its possible use or anything like that.

It is an underappreciated fact (and one that I hope my book on this subject — the history of nuclear secrecy — will cause to be more appreciated) that the secrecy of the Manhattan Project was more poised at preventing American institutions (the press and Congress) from knowing about the work than it was to preventing the Germans or the Russians. Why? Not for some nefarious conspiratorial reason, but for a very practical one. Only the press and Congress could actually shut the project down, and they were desperately afraid of that for the entirety of the project, for good reason.