There should be one project technical officer (not the officer in charge of general management, including money transfering, invoicing, ...) from DOD, who supervises your project. I think your PI can ask this person to write a letter, but not sure if he can wirte one. At least you can ask your PI to give you some documents describing the importance of your project. Such documents may include: reviewer's comments on your project, project summary published in DOD website, supporting letter for your project from independent person ...
Good luck!