Serving as a testbed to study the retrofit options for the CFM56 engine on the 707. After testing in 1979, the aircraft was refitted as a 707-320C. Boeing abandoned the program over fears of hurting 757 sales. However, the program however did lead to military aircraft being retrofitted. To the Morrocan Government in March 1982 as CN-ANR, reregistered CN-CCC in January 1999. To Omega Air in January 1999 as N707JJ. To Israel Aircraft Industries as 4X-980 in February 2000. To the Israeli Air Force as 290 in October 2000. Still active at the time of the upload. Photo from Boeing