Later that year an enlarged version of the Comet finally returned to service, but too late to take a substantial portion of the market: de Havilland had just 25 orders. In August Boeing had begun delivering 707s to Pan Am. Douglas made a massive effort to close the gap with Boeing, using no fewer than ten aircraft for flight testing to achieve FAA certification for the first of the many DC-8 variants in August 1959. Much needed to be done: the original air brakes on the lower rear fuselage were found ineffective and were deleted as engine thrust reversers had become available; unique leading-edge slots were added to improve low-speed lift; the prototype was 25 kn (46 km/h) short of its promised cruising speed and a new, slightly larger wingtip had to be developed to reduce drag. Also, a recontoured wing leading edge was later developed to extend the chord 4% and reduce drag at high Mach numbers.