Military overflights could have been considered an "act of war." Civilian pilots flying weather research airplanes for NACA (now NASA) and "got lost" have deniability.

In fact the CIA U2 pilots were "military" who had been "sheep dipped." Paperwork said they were former military pilots. Interestingly any who chose to remain in the military did not loose any time in grade for promotion or towards retirement during the years they flew for the CIA.

SHEEP-DIPPED - Stripping a soldier of his military uniform and identification so he can pose as a civilian during a covert mission.

A classic example is Hervey Stockman who flew the first CIA overflight on July 4, 1956. He was a WWII fighter pilot and continued with his USAF career into the Vietnam War. His F4 was shot down and he was a POW in the Hanoi Hilton for 6 years. The cover for his CIA U2 years was so good the Soviets never knew the pilot of the first U2 overflight was sitting in the Hanoi Hilton.

R.I.P. Hervey Stockman, First Pilot to Fly a Spy Plane Over Soviet Territory