Does the Air Force (United States) help design their own planes?
While the Air Force (or other military unit) does not design their own jets, they do release specifications and have contests (e.g. the ATF that selected the F-22). This is basically the way it works (and not just for fighter procurement):

A design specification is released, these include things like range, mission, weapons.
A &quot;paper&quot; design is submitted from companies like Boeing and Lockheed Martin. These may be based off of existing designs or &quot;clean sheet&quot; designs.
The military funds the development of one or two prototype aircraft from a number of paper designs.
The military evaluates each design against the specification and selects a &quot;winner&quot;.
A larger contract is awarded

So it's not really up to companies like Boeing or Lockheed Martin to come up with their own designs and sell that to the military. They work under strict program requirements to develop something &quot;to spec&quot; and then demonstrate the capability. The military has a hand in the design process as this goes along, so they are not entirely &quot;out of the loop&quot;.
The secondary market is different though. Boeing may sell (with the approval of the US Government) these aircraft to certain foreign military groups. These work more like selecting something from available stock in the dealer lot.