Editing Boeing 2707 (section)

===Design competition===
Requests for proposals were sent out to airframe manufacturers Boeing, [[Lockheed Corporation|Lockheed]], and [[North American Aviation|North American]] for the airframes; and [[Curtiss-Wright]], [[General Electric]] and [[Pratt & Whitney]] for engines. The FAA estimated that there would be a market for 500 SSTs by 1990. Despite not having a selected design, orders from air carriers started flowing in immediately.<ref>{{cite magazine |url=http://www.time.com/time/magazine/article/0,9171,875695,00.html |title=Aviation: SSTart |magazine=[[Time (magazine)|Time]] |date=January 24, 1964 |archive-url=https://web.archive.org/web/20130823161441/http://www.time.com/time/magazine/article/0%2C9171%2C875695%2C00.html |archive-date=August 23, 2013}}</ref> Preliminary designs were submitted to the FAA on January 15, 1964.<ref name="contenders">{{cite web |url = http://www.flightglobal.com/pdfarchive/view/1964/1964%20-%200377.html |title = The United States SST Contenders |work = [[Flight International]] |date = February 13, 1964 |pages = 234–235}}</ref>

[[File:NAC-60 model side view.jpg|thumb|The NAC-60 was North American's entry. It retains a number of features from the B-70 it was based on, notably the high-mounted canard behind the cockpit area, and details of the engine area under the rear fuselage.]]
Boeing's entry was essentially identical to the swing-wing Model 733 studied in 1960; it was known officially as the Model {{not a typo|733-197}}, but also referred to both as the 1966 Model and the Model 2707. The latter name became the best known in public, while Boeing continued to use 733 model numbers internally. The design resembled the future [[B-1 Lancer]] bomber, with the exception that the four engines were mounted in individual nacelles instead of paired pods used on the Lancer.<ref name=contenders/> The blended wing root spanned almost all of cabin area, and this early version had a much more stubby look than the models that would ultimately evolve. The wing featured extensive high-lift devices on both the leading and trailing edges, minimizing the thrust required, and thus noise created, during climb out. The proposal also included optional fuselage stretches that increased capacity from the normal 150 seats to 227.

Lockheed's entry, designated CL-823, was essentially an enlarged Concorde. Like the Concorde, it featured a long and skinny fuselage, engines under the wing, and a compound delta planform. The only major design difference was the use of individual pods for the engines, rather than pairs. The CL-823 lacked any form of high-lift devices on the wings, relying on engine power and long runways for liftoff, ensuring a huge noise footprint. The CL-823 was the largest of the first-round entries, with typical seating for 218.<ref name=contenders/>

The [[North American NAC-60]] was essentially a scaled-up [[North American XB-70 Valkyrie|B-70]] with a less tapered fuselage and new compound-delta wing. The design retained the high-mounted canard above the cockpit area, and the box-like engine area under the fuselage. The use of high-lift devices on the leading edge of the wing lowered the landing angles to the point where the "drooping nose" was not required, and a more conventional rounded design was used. Compared to the other designs, the rounded nose profile and more cylindrical cross-section gave the NAC-60 a decidedly more conventional look than the other entries. This also meant it would fly slower, at Mach 2.65.<ref name=contenders/>

A "downselect" of the proposed models resulted in the NAC-60 and Curtiss-Wright efforts being dropped from the program, with both Boeing and Lockheed asked to offer SST models meeting the more demanding FAA requirements and able to use either of the remaining engine designs from GE or P&W. In November, another design review was held, and by this time Boeing had scaled up the original design into a 250-seat model, the ''Model 733-290''. Due to concerns about jet blast, the four engines were moved to a position underneath an enlarged tailplane. When the wings were in their swept-back position, they merged with the tailplane to produce a [[delta-wing]] planform.

Both companies were now asked for considerably more detailed proposals, to be presented for final selection in 1966. When this occurred, Boeing's design was now the 300-seat ''Model 733-390''. Both the Boeing and [[Lockheed L-2000]] designs were presented in September 1966 along with full-scale [[mock-up]]s. After a lengthy review the Boeing design was announced as the winner on January 1, 1967.<ref name=comm/> The design would be powered by the [[General Electric GE4|General Electric GE4/J5]] engines. Lockheed's L-2000 was judged simpler to produce and less risky, but its performance was slightly lower and its noise levels slightly higher.