the first boeing yc-14 (serial number "72-1873") flew on 9 august 1976. two aircraft were built, the second being s/n "72-1874". the competing yc-15 had started flights almost a year earlier. head-to-head flight testing at edwards air force base started in early november 1976. during flight testing, the yc-14 was flown at speeds as low as and as high as mach0.78 at . however, it was found that the yc-14's drag was 11% higher than originally predicted. modifications developed in wind tunnel testing, comprising the addition of vortex generators to the upper aft portion of the nacelles, deletion of the nozzle door actuator fairing, alterations to the aft end of the landing gear pods and the addition of aft fuselage strakes, reduced this drag increment to 7%. the yc-14 also demonstrated the capability to carry the m60 patton main battle tank, something that was not demonstrated with the yc-15.