This question comes from an interview I conducted many years ago. The question about drafts reminded me. The subject was a grad student and medical researcher. His research project was canceled and he was assigned to the early stages of the Manhattan Project, which first started at the university level. He worked on the project until the end of the war.
According to the Selective Service, defense contractors were a form of deferment and thus were not drafted. That sounds like someone should be a defense contractor first, but this subject was not. His civilian assignment was non-voluntary.
This Wikipedia page gives a little bit of a clue as to a possible mechanism:
Industry realized that the Army urgently desired production of essential war materials and foodstuffs more than soldiers. (Large numbers of soldiers were not used until the invasion of Europe in summer 1944.) In 1940–43 the Army often transferred soldiers to civilian status in the Enlisted Reserve Corps in order to increase production. Those transferred would return to work in essential industry, although they could be called back to active duty if the Army needed them. Others were discharged if their civilian work was deemed essential.
Does anyone know how a lower level scientist or other support staff were brought on to the Manhattan Project specifically? Was there a way for the government to target specific kinds of skill sets, for instance, similar to the "doctor draft" instituted during Korea. And was it common during WWII to use non-voluntary "deferments" for strategically critical civilian personnel?
Note: The other possible mechanism I can think of is that his grant was cancelled from war-time cut-backs and he was offered the job on another project with the government, even though he didn't know what it was and this prevented his possible drafting. The interview is unclear. Any information is appreciated!