Red Hat is pretty much the only Linux distro used in military systems. Purpose-built military information systems, like the Distributed Common Ground System (Army/Air Force) and (parts of the) SPY-1D radar system (Navy), generally use Linux for any purpose that does not require hard real-time guarantees.

Of course, they never really used Windows for this purpose at any point; Linux is gradually taking over for Solaris on operator consoles and AIX on the heavy processing machines.