Expedia shows YX as the ns option between MKE and DCA; it also shows that there are no ns flights for MKE-BOS, though.

Certain other YX ns routes, such as MKE-PHX and MKE-LAS show up as codeshares with NW (the carrier is shown as NW, with the small facing arrows alongside the carrier indicating codeshare); the flights are not listed as YX. It appears that NW codeshares on YX routes will only appear where NW does not provide ns service itself.