When modified permanently for special tests, a number of T-33As was redesignated NT-33As. Among these aircraft was [51-4120], which was successively used for tests by Lockheed, the Allison Division of General Motors, the USAF Wright Air Development Center, and the Flight Research Dept of the Cornell Aeronautical Lab (later reorganized as Calspan Corp).

With Calspan the aircraft was fitted with a larger-volume nose section from an F-94A to accommodate test equipment and computer electronics. Used for variable-stability research and for simulating the flying qualities of a variety of aircraft then under development, this NT-33A was at various times fitted with alternative controls (wheel, stick, two- and three-axis side controller), "fly-by-wire" systems, modified tip tanks with electrohydraulically-operated drag petals, and in-flight refuelling probe. In 1980 Calspan was still using the ship in a project sponsored jointly by USAF and USN, known as Display Evaluation Flight Test (DEFT).

Two other NT-33As were [48-357] and [51-4263]. The former—the second TF-80C—was used for ejector-seat trials and fitted with aft-focusing camera recording equipment on top of the nose and with an open rear cockpit. The latter was modified by Lockheed to have twin fins and rudders at the ends of the tailplane instead of the standard single-tail surfaces. The modified tail was at the time considered for the proposed TV-2 carrier landing trainer. Testing was not fully satisfactory and the new aircraft, model L-245 (USN T2V-1), had conventional tail surfaces. (— Johan Visschedijk 9/10/03)