US Joint Chiefs of Staff General Joseph Dunford attended on Pentagon's 2020 Budget at the House Armed Services Committee. General Dunford stated that F-35 is costlier than they had predicted and therefore they had to acquire more F-15 to balance the need and expenses. Dunford said, “If you could buy all f-35s, you might do that. But there's a 50% difference in the operations and sustainment costs between the F-15 and F-35. The F-15 has a significant shelf life available as well. So it was the combination of the platforms that we made a decision on.”