William Blum in Killing Hope argued that:
How substantiated is this assertion?
There were many other CIA flights over China for purely espionage purposes, carried out by high-altitude U-2 planes, pilot-less "drones", and other aircraft. These over-flights began around the late 1950s and were not discontinued until 1971, to coincide with Henry Kissinger's first visit to Peking. The operation was not without incident. Several U-2 planes were shot down and even more of the drones, 19 of the latter by Chinese count between 1964 and 1969. China registered hundreds of "serious warnings" about violations of its air space, and on at least one occasion American aircraft crossed the Chinese border and shot down a Mig-17.
How substantiated is this assertion?