Allied Signal modified a 720-051B
to test the TFE731-40 turbofan engine.
Allied Signal's TFE731-40 Testbed wears construction number 18384. It first flew on September 15, 1961. TWA leased it as N794TW beginning on August 27, 1961. Northwest leased it as N733US on October 26, 1962 and then bought it on July 1, 1968. Maersk Air registered it as OY-APZ in January 1973 and leased it to Nigeria Airways in December 1974. TEA leased it as OO-TYA on November 6, 1979 and returned it to Maersk on January 19, 1980. Conair acquired it on February 16, 1981.
Allied Signal registered it as N720GT on November 24, 1987 and mounted an engine test pylon on the right side of the forward fuselage.