the tunnel was completed in 1936 at a cost of $36,266,000. because of its high speed and bernoulli's principle, the pressure in the test section is much lower than that in the rest of the tunnel. this required a structure that could withstand an inward force due to the pressure difference. instead of steel construction, it was built from reinforced concrete with walls up to 1 ft (0.3 m) thick. this resulted in an "igloo-like" structure at the test section. the wind tunnel was designed as a single-return tunnel capable of moving air at speeds up to a mach number up to 0.75. it was powered by an electric motor. it was repowered to to give mach number 1 capability in 1945. in 1947, the speed was increased to a mach number of 1.2 with the installation of a contoured nozzle. in 1950, a slotted-throat test section was installed, and it was repowered to .