The support requirements for Apollo, compiled by NASA’s Office of Manned Space Flight (OMSF), originally stated a need for twelve heavily instrumented, long-range, high-speed aircraft to supplement the telemetry and communication support to be provided by the Apollo ships. It was decided after program refinement that the aircraft requirement would be eight, six to be on station with two standby spares.
Modification of the eight C-135s, which was extensive because of the shift in the center of gravity, was performed by Douglas Aircraft Modification Center in Tulsa Oklahoma. Douglas was the prime contractor for the A/RIA program and Bendix Radio is electronic subcontractor.
Source: ARIA Archive