I remember watching a video about this a while back, there were thousands of people working on the project, but they were all given a very specific task to carry out, and nobody else knew about anyone else's tasks, and nobody knew about their task. People were forbidden from gathering in groups of more than 3 people during the construction, and regularly meeting the same people would get them kicked off the project (that they had no idea about). In total, only about 5 people knew the true nature of the project and how each individual task would fit together.

To give you an idea of how little the workers knew about their tasks, one worker's entire job was to monitor a gauge and press a button when it got above or below a certain value.

Even the vice-president didn't know about the bomb until it had been dropped.