You don't need to build it to test volume and aerodynamics (math and wind-tunnels can figure that out for you). This is probably more about finding unknown flaw

You don't need to build it to test volume and aerodynamics (math and wind-tunnels can figure that out for you). This is probably more about finding unknown flaws in the design/running down risk. Spending however much on a prototype (10s or 100s of millions of dollars) just to marginally reduce the risk that there's a fundamental flaw in your approach makes a lot of sense when you plan to spend 6 billion total to develop the plane.