I did some informational interviewing at Boeing's Everett offices about 18 months ago, and I can tell you the Sonic Cruiser was not a PR
gimmick. The walls were covered with detailed development process flow charts, status reports, progress targets, systems evaluations, and so on. I could tell that tremendous effort had already gone into the planning process. Hundreds of people in that one little corner of Boeing were working on the Sonic Cruiser all day, every day. I have a picture of me standing in front of an 8-foot Sonic Cruiser model, surrounded by renderings and even a painting of the plane - and this was in everyday offices, not a showroom. If the SC
was a smokescreen whipped up by management to distract Airbus and the airlines, Boeing went to extremes to trick their own employees as well.
It would be awesome to see the 2707 fly. This was a truly large aircraft, as shown below in a size comparison with the 747-400. The sheer size of it (306 feet long) would dwarf even the A340-600, which is today's longest aircraft at 246 feet 11 inches.
With modern computational fluid dynamics technology and light-weight composite materials, I do not doubt that the 2707 could now meet its design goals of carrying 300 pax over 4,400 miles. The problem is that the 2707 was designed to be more efficient than Concorde and the 707-320B, but modern aircraft have advanced so much that it would still be obsolete.
How much the 2707 design could be updated, and whether the result would be economically competitive, is something I have been looking into for a long time. A clean-sheet SST design would almost certainly be more efficient, if somewhat more costly. But when you compare NASA's High-Speed Civil Transport design goals with the 2707-100, it's amazing how many of the basic parameters Boeing got right: pax load, fuselage cross-section, aircraft weight, fuel capacity, engine size, wing area, etc. There has even been talk of bringing back the variable-sweep wing to help solve noise problems, which continue to plague the HSCT, by increasing takeoff and landing performance. Unfortunately for the US, the technology of the 1960s could not support such an ambitious project.
Keynes is dead and we are living in his long run.