Boeing 707 center section tanks. Its first flight was on January 11, 1958; 69 turbojet 707-320s were delivered through January 1963, the first passengers being carried (by Pan Am) in August 1959. The 707-420 was identical to the −320, but fitted with Rolls-Royce Conway 508 (RCo.12) turbofans (or by-pass turbojets as Rolls-Royce called them) of 18,000 lb (79 kN) thrust each. The first announced customer was Lufthansa. BOAC's controversial order was announced six months later, but the British carrier got the first service-ready aircraft off the production line. The British Air Registration Board refused to give the aircraft a certificate of airworthiness,
Boeing but the design experience was used in the development of the Model 314 that followed a year later. Overlapping with the period of the YB-15 development, an agreement with Pan American World Airways (Pan Am) was reached, to develop and build a commercial flying boat able to carry passengers on transoceanic routes. The first flight of the Boeing 314 Clipper was in June 1938. It was the largest civil aircraft of its time, with a capacity of 90 passengers on day flights, and of 40 passengers on night flights. One year later, the first regular passenger service from the U.S.