07/15/2006. Jack McKillop: "This aircraft was purchased by the USAAC for use as a high altitude research and pressurized cabin test plane and was the world's first airplane specifically constructed with a pressure cabin. The basic Model 10 fuselage was redesigned with a near circular cross section to better withstand the stresses of pressurization and the large passenger windows were replaced with much smaller slit windows.
The interior was split into two sections, the forward pressurized section had room for three crewmen and two passengers; the aft section, behind the pressure bulkhead had room for one additional passenger but could only be used at lower altitudes below 12,000 ft (3,658 m). Besides the pilot and copilot, the XC-35 carried an engineer who controlled the pressurization and high altitude research equipment.
Power was provided by two 550 hp (410 kW), nine-cylinder, single row, air-cooled, turbosupercharged Pratt & Whitney XR-1340-43 radial engines. The plane was designed to fly at altitudes above 30,000 ft (9,144 m)."
Read also the type remarks in page 5752.