The MX-1018 program (code named "Tip Tow") sought to extend the range of the early jets in order to give fighter protection to piston-engined bombers with the provision for in-flight attachment/detachment of the fighter to the bomber via wingtip connections. The Tip Tow aircraft consisted of a specially modified ETB-29A (serial number "44-62093") and two EF-84D (serial numbers "48-641" and "48-661"). A number of flights were undertaken, with several successful cycles of attachment and detachment, using at first a single aircraft, and then two. The pilots of the F-84s maintained manual control when attached, with roll axis maintained by elevator movement rather than aileron movement. Engines on the F-84s were shut down in order to save fuel during the "tow" by the mother ship, and in-flight engine restarts were successfully accomplished.