Re: USN P-51D - added reworked F1J-4 and F1J-5...
Ryan Crierie provided me with some BuAer and NAA documentation on the NA-133. In summary, there was a Navy military requirements conference in July 1944 which produced "performance characteristics for future VF (fighter) types". It specifically requested that proposals be solicited for a liquid-cooled engine interceptor that met those requirements, probably over continued concern about the kamikaze threat. The requirements seem unlikely to be fulfillable: among other things, the time to climb to 30,000 feet was only five minutes, quicker than the F8F Bearcat or F2G could get to 20,000 feet. The BuAer engineering division responded a month later with a request for more specifics on the request, such as armament and armor details, engine ratings to be used for the interceptor climb requirement, stall speed, etc. Reading between the lines, the response suggests that the engineering division wasn't impressed by the feasibility of the product from the conference. The response also stated that the proposal request would not be limited to liquid-cooled engines, but would solicit air-cooled engine designs and ones with a piston engine combined with a jet.
At the same time as the engineering division response, George Spangenberg appears to have drafted a letter that the BuAer Chief sent to the Chief of Naval Operations stating that the Bureau would solicit proposals for a liquid-cooled engine fighter but comparative studies "indicated that the use of an air-cooled engine resulted in a superior airplane to meet current carrier requirements." In any event, the Grumman F8F Bearcat had just flown, representing the pinnacle of interceptor performance with existing supercharging.
The North American Detail Specification is dated 14 October 1944. It includes a simplified three-view drawing basically identical to the one that Greg posted: The tail hook installation relative to the rudder is a bit different and the position of the wing when folded is not shown. North American also submitted a proposal for a fighter, NA-134, powered solely by a jet engine. BuAer elected to contract for the jet as the FJ-1, although the NA-133 design was considered "very attractive in certain respects."
It now is clear that the P-51D evaluation aboard Shangri-La in November 1944 was in support of this activity.