DEVELOPMENT OF NEPA/ANP FOLLOWED THE LEXINGTON REPORT OUTLINE FROM 12 MAY1948

Stever, H. G. Minutes of Meeting Held May 12 and13, 1948 Between Representatives of NEPA and Project Lexington and AEC.. United States: N. p., 1948. Web.

Document from 12-13 May 1948 Lexington Project Meeting Minutes 

The timeline shows that work on the direct nuclear turbojet was completed/perfected in 1951. It produced the Canberra. Next thing work began on the indirect system. It produced the U-2 in 1956. This is when development of the nuclear ramjet and rocket began. In other words this document shows that the nuclear turbojet in both it's direct and indirect forms must have had been perfected by 1956. 

We know from the 25 November 1949 UFO sighting over Mt. Palomar that a nuclear plane was being flight tested in Southern California. Probably a sighting of the NB-36. By 1951 the Canberra was modified to fly above 70,000 feet and run on nuclear power. In 1952 it was overflying the Soviet Union. In 1956 the nuclear U-2 was deployed. 1956 is when the development of the nuclear ramjet began under Project PLUTO. In 1959 the experiments on the Nuclear Rocket began under Project ROVER. 

We know from the 1949 LA Times article that they were already looking at high altitude planes for the NEPA program. 

LOS ANGELES TIMES 27 SEPTEMBER 1949