Yes it was, at least it was for a several decades.
Boeing gave the KC-135 the model number 717. The 707 and the KC-135 derived from the same prototype, the 367-80, but the 707 has a wider fuselage (a distinctive "double bubble"), and is overall larger than the KC-135.
From the July 2006 issue of Boeing Frontiers (internal Boeing magazine):
"The Dash 80 led to two airplanes: The 707, the world's first successful commercial jet; and the Model 717, the world's first production jet tankerbetter known as the KC-135 (717 was also used as the product designation for the MD-95 after the BoeingMcDonnell Douglas merger)."
Denoting the KC-135 as Boeing model 717 happened early in the airplane's career, because when Boeing built the smaller, higher performing model 720 airliner to compete with the Convair 880 and 990, Boeing named it the 720 (an amalgamation of 707-020) because the 717 model number had already been used for the KC-135.
So, the 717 designation has been applied to two different planes -- the KC-135 and the MD-95 (or DC-9, as it used to be).
Thanks for the clarification. It seems that 717 was an internal Boeing designation. We alway roughly equated the -135 to 707s.