That's Super Frog - ATT story in words of Boeing:
"In 2000, Boeing initiated in-depth configuration design
studies to address future theater airlift mobility needs.
Numerous configurations were explored, and quickly a
four engine, tilt-wing, propeller design with Super
Short Takeoff and Landing (SSTOL) performance was
determined to be the best integrated design solution for
an Advanced Theater Transport (ATT) mission. Little
was it realized the role that 1960-70’s research would
play in the validation of the Boeing ATT concept.
Boeing’s early tilt wing concept, developed in the late
1990s was a tail-less design called the Super-Frog. The
performance was computed using Boeing’s theoretical
powered lift aerodynamics methods, which calculated
lift and drag based on thrust coefficient and geometric
parameters. Trimmed flight analysis was limited with
these methods. As detailed design work was
commenced in 2000, other databases and
methodologies were investigated to improve pitching
moment predictions and validate Boeing’s empirical
high lift methods. It became evident that the last
significant amount of research on tilt-wing propeller
designs was concluded in the early 1970s. The USAF
Light Intratheater Transport (LIT) studies in 1968 were
a valuable source for tilt-wing design methodology.
Numerous tilt-wing designs that flew in this period,
including the Hiller X-18, the Canadair CL-84, and the
Ling-Temco-Vought XC-142 also provided valuable
information. But, the most complete and thorough
source of powered lift data that was relevant to
Boeing’s ATT was NASA’s powered-lift, tilt-wing,
wind tunnel database. This included small scale wind
tunnel testing in NASA facilities and large scale wind
tunnel testing in the NASA-Ames 40 X 80 ft wind
tunnel.
The reliance upon this research conducted over 30 years
ago allowed Boeing to quickly establish a baseline
design, and proceed to a sophisticated 11% powered
wind tunnel model with a high degree of confidence in
the predicted lift, drag, and pitching moments. In
addition, there were many lessons learned from the tiltwing
XC-142 that were incorporated into the design of
Boeing’s 21st century tilt-wing design."
from
AIAA 2002-6023
DESIGN AND DEVELOPMENT OF A SUPER-SHORT TAKEOFF AND LANDING TRANSPORT AIRCRAFT
David J. Manley
Program Manager - Boeing Advanced Theater Transport
Huntington Beach, California
Walter von Klein, Jr.
Principal Engineer, Boeing Phantom Works
Huntington Beach, California