For reference, 1.5 trillion is more than double the Apollo and Space Shuttle programs, combined.
Look what happened to the Airbus A400M. They agreed to a fixed price, and then when the program ran away over budget they blackmailed the EU defense ministers into renegotiating by threatening to cancel the program, close factories, or even go bankrupt. It's just politically impossible to hold the line on these deals.
But there have been some contracts (NASA commercial cargo to the space station) that have had fixed milestones for releasing money to contractor.
Big .gov orgs have a lot of leverage, because they can make or break the sales force.
The end result is that the vendor prices in the risk and overhead. Depending on the political situation and project, it may be worth paying that premium to displace the blame to the vendor. But for something like the F35, the cost is astronomical.
IMO the F-35 is fine from a capability standpoint. It's just far too expensive.
frankly i'm sure that if it came to it, your average clip on tie beltway bandit employee would be more useless than 5 year old in time of war, but i think thats the thinking