Most modern fighter jets are designed to operate up to around 65,000 feet, although some go higher. It is extremely difficult, if not impossible, to put an accurate list together about how high military aircraft fly. The United States Air Force often discloses the altitude below its actual. For example, the Lockheed U-2 has an unclassified altitude of 70,000 feet, but it is set to appear as 60,000 feet on flight trackers, regardless of whether it’s higher. It is known to have flown at over 74,000 feet, with reports suggesting it can reach 80,000 feet.