It was very much top-down. Most of the production was nominally done by private contractors, but the federal government forced existing factories to be converted to war production, fronted the money to build new factories, directly built public housing for the factory workers, and fixed wages and consumer prices.
> existing factories to be converted to war production
And some amusing artifacts were produced as a result of that. For example, the M1 Carbine was manufactured among others by IBM, Rock-Ola, and the National Postal Meter Company. After the war, these became collector items of sorts, since the manufacturer name is prominent on all of them.
