In a couple of recent conversations, I've heard folks invoke the Manhattan Project as a demonstration that given a vigorous enough government commitment, scientists can be directed to secretly invent the seemingly impossible. Unfortunately, that's just not true.
I think a lot of people imagine that one sunny day in 1942 the US government came to the scientific community and said, “we need a city-busting bomb, and we'll give you resources to do it,” and the scientists quietly whipped up the Bomb to order. But that is the reverse of the way it happened.
Physicists had been designing atom bombs on blackboards as a “thought experiment” since the 1930s. Seeing the theoretical possibility of the Bomb wasn't hard to figure out. It was the scientists who came to the government, aware of the potential to make the Bomb and horrified by the prospect that the Germans might develop it leaving the Allies at their mercy. It wasn't the conceptual difficulty of the Bomb which necessitated the mammoth effort of the Manhattan Project. Yes, having a large team of scientists did help do it more quickly, and resulted in the team delivering two different successful Bomb designs. But it was the materials problem of separating the uranium isotopes that required the magnitude of the Manhattan Project. Scientific discovery and technological invention do scale with investment, but only to a degree.
And as a secret effort, the Manhattan Project may be unique. Obviously we still have secret military research in the US, and totalitarian nations can conduct big secret projects, but it's difficult to imagine keeping something as big as the Manhattan Project under wraps again. Scientists tend to be garrulous and iconoclastic; only the uniquely galvanizing threat of Hitler with the Bomb could have kept so many of them both engaged and careful to keep the secret.