Page 29 - Volume 15 Number 1

when the decision was made to design a low-wing, tandem-seat monoplane as a potential replacement for the venerable Model 75 series biplanes. As a result, Project 18A was initiated to create an airplane that would utilize a maximum number of parts and assemblies from the PT-13 and PT-17. Boeing assigned 26 engineers to the program, and based on encouragement from the Army Air Corps, by September 1939 a prototype designated the Model X90 was to be built on speculation.2
To minimize the use of strategic materials needed for the war effort, the X90 featured a wood wing and empennage and the forward fuselage section used welded steel tubing. The aft fuselage was of all-metal, semi- monocoque construction. The conventional main landing gear was fixed and the cockpit was covered by a sliding canopy. A Lycoming R-680 radial engine rated at 225 horsepower was chosen for the prototype, although the airframe was stressed to accept engines of up to 450 horsepower.
The prototype was ready for its first flight Nov. 1, 1940, and initial flight tests were conducted that month at the Wichita municipal airport. One month later, on December 1, 1940, the Air Corps requested demonstration flights at Wright Field and “Deed” Levy delivered the ship as planned. At Wright Field the X90 underwent a series of evaluations, including replacement of the Lycoming powerplant with a 450-horsepower Pratt & Whitney R-985-AN-1 radial engine. When the Air Corps completed its work, the airplane was flown to Naval Air Station Anacostia near Washington, D.C., where it was demonstrated to the Navy. Both the Army and Navy liked the X90 and recognized its potential as a replacement for the PT-13 and PT-17.
The monoplane trainer returned to Wichita in February 1941 and the original flight test program outlined by Boeing resumed. Unfortunately, the X90 was introduced at a time when war clouds were gathering over Europe and the Stearman Division’s rough-and- ready PT-13 and PT-17 biplanes were already in mass production and providing excellent service in the field. Taking these factors into account, the War Department lost interest in the new trainer.
In May 1941, however, Boeing and the Army Air Corps opened negotiations that resulted in a contract for a basic training version designated the XBT-17. Modifications specified by the Air Corps were made to the prototype X90 and Boeing delivered the XBT-17 to the Army in January 1942. By that time, the United States was at war with the Axis Powers and once again, the War Department’s deepening concerns about a shortage of aluminum alloy and other materials, led to the Army’s decision not to place orders for the XBT-17.3
JANUARY 2021
KING AIR MAGAZINE • 27