The B-17 "Flying Fortress" was supposed to be so well-armed that formations of them could fly over enemy territory with near impunity, killing off enemy fighters as easily as bug zappers fight off flies. But the reports of B-17 immunity were drastically overstated, and aviators paid the cost by the hundreds before a solution was deployed.
In the lead up to World War II, the U.S. Army Air Force had to make tough decisions on how to spend limited defense dollars. Decades of strict budgets after World War I left capabilities across the military underdeveloped, and the Air Forces decided to spend their part of the pie focusing on strategic bombing.
And, unfortunately, when a manufacturer told them a new bomber wouldn't need a fighter escort, they bought it. Thousands of aviators would pay the price as unescorted B-17 formations faced losses of over 20 percent.