The Model 4 Full Pressure Suit was developed for D-558-2 Douglas Skyrocket test pilots. It was first flown by Navy test pilot Marion Carl for a 26 km altitude record flight. Integrated arm scye bearings (non-sealed) provided improved mobility. Custom sizing was required to tailor the suits to individual test pilots. This was the forerunner of the X-15 full pressure suit and was developed by the David Clark Company and Scott Crossfield at NACA.