Strictly speaking, the US DoD doesn't have plants/factories of its own anywhere.
It's various sub-agencies (DARPA, NRO, NSA, CIA, etc) create contract opportunities that private companies bid on.
The government's role is in deciding contracts, dispersing money for payments, determining compliance and staffing the administrative operations. The actual 'stuff' is almost always built by private companies.
Assuming you're referring to the location of where those private contracts are handed out, it's more interesting to look at how many contracts there are per Congressional District, not how many plants/factories. If it's about paying for Congressional votes, a physical "plant" doesn't have to be there. Money just has to flow.
In 2011, there were DoD contracts awarded in all 435 Congressional Districts.
That's not surprising when you consider:
a) The DoD annual budget is some $600 billion per year
b) There are over 170,000 government contractors in the US, of which about 77k work primarily for the DoD
c) The average size of a Congressional District is pretty big - more than 640k people. What are the odds that with a defense budget that large, a random group of 640k people isn't going to end up with a piece of the pie in one way or another.