An interesting lost airplane that ended squeezed (and screwed) between RB-57D and U-2 when it had characteristics of both...
Based on what I've read, the airplane was more of an Air Force-favored also-ran squeezed out by a superior, CIA-favored U-2. The design of the two aircraft and the eventual selection of the U-2 reflected opposed USAF and CIA approaches to overflight of denied territory--one military and dangerously provocative, the other civilian and seemingly less so. The latter prevailed.
The Air Force had long favored converted strategic bombers for strategic reconnaissance and wanted at least a medium-bomber-sized, mil-standard airplane with room for growth. Its initial favorite, the Canberra-based RB-57D, proved both unable to reach the required altitudes and overly fragile. The twin-engined X-16 was essentially an improved B-57-class aircraft with the strength required to do what its predecessor could not.
The USAF's bomber mindset extended to the tactics it adopted. Under Curtiss LeMay, the USAF made a habit of blatantly and illegally sending its B-45s, B-36s, and B-47s into Soviet airspace, often
en masse, with little effort at concealment. These dangerous overflights were not authorized by the US President or Congress and so deliberately provocative that some speculate that LeMay was actually trying to provoke a nuclear war so that he could win it.
As a clandestine intelligence service, the CIA naturally took a different approach to the problem. It proposed clandestine and, if necessary, deniable overflights by single, civilian-registered aircraft, each flown by a single, civilian, ideally foreign-born contract pilot. The U-2 airplane itself was designed to fly undetected at altitudes far in excess of the supposed maximum height capability of Soviet radars. The designers of the sailplane-like structure ignored normal military requirements in single-minded pursuit of light weight. They allowed weight and volume for the absolute minimum required intelligence payload. The result was emphatically
not a combat aircraft, at least by then contemporary standards. If it
were detected, the CIA claimed, it just
might pass as an errant research plane, something that a bomber-like X-16 in USAF SAC markings could not do.
Unlike LeMay, US Presidents were not anxious to openly violate international law or commit acts of war. So I suspect that the CIA's U-2, with its (ill-founded) promise of undetectability and its less provocative, unmilitary appearance, held the edge over the X-16 as soon as Lockheed proposed the latter.