Some of the most famous scientists and mathematicians who worked on the Manhattan Project to develop nuclear warheads were Albert Einstein, Enrico Fermi, Richard Feynman and Niels Bohr. However, the full staff list ran to over 120,000 people, many of whom did not know the true nature of the project.
As a full-scale research and development project during World War II, the Manhattan Project required a well-trained and equipped staff at all times. Many of the scientists who are most famous for working on the Manhattan Project were, however, mainly expert advisers on nuclear power and mathematical modelling. Therefore, they had little to do with the day-to-day development of the nuclear weapons, which was tasked to engineers and demolition experts.
Because of the need to keep the development work from the Axis powers and Imperial Japan, a culture of extreme secrecy developed. The U.S. and U.K. leaders arranged the project in such a way as to prevent public awareness or inquiry. Managing that level of secrecy with 120,000 people who knew the inner workings of the project seemed impossible, so despite practical difficulties, most of the staff working on the project were kept unaware of its true nature until the nuclear age had already began.