16 August 1948 – “The Scorpion stemmed from a United States Army Air Forces (USAAF) Air Technical Service Command specification (“Military Characteristics for All-Weather Fighting Aircraft”) for a night fighter to replace the P-61 Black Widow.” (1) The specifications called for the aircraft to fly at a “desired maximum speed of 530 miles per hour” which demanded that it would need to be jet-powered.
“The inspectors believed that the radar operator needed to be moved forward, closer to the pilot, with both crewmen under a single canopy, the magnesium alloy components of the wing replaced by aluminum alloy, and the fuel tankage directly above the engines moved. Other changes had to be made as wind tunnel and other aerodynamic tests were conducted. The swept wings proved to be less satisfactory at low speeds, and a thin straight wing was selected instead. Delivery of the first prototype was scheduled for November 1947, 14 months after the inspection. The position of the horizontal stabilizer also proved to be unsatisfactory, as it was affected by the engine exhaust, and it would be “blanked-out” by airflow from the wing at high angles of attack. It was moved halfway up the tail, but its position flush with the leading edge of the vertical stabilizer proved to cause extra drag through turbulence and reduced the effectiveness of the elevators and rudder. Moving the horizontal stabilizer forward solved the problem. Another major change occurred when USAAF revised its specification to delete the rear gun installation on 8 October. Another inspection of the mock-up was held on 17 December, and the inspectors suggested only minor changes, even though the fuselage fuel tanks were still above the engines. Northrop’s efforts to protect the fuel tanks were considered sufficient, as the only alternative was to redesign the entire aircraft.
The XP-89 had a thin, straight, mid-mounted wing and a crew of two, seated in tandem. The slim rear fuselage and the high-mounted horizontal stabilizer led Northrop employees [to call] it the Scorpion—a name later formally adopted by the Air Force. A month before the prototype made its first flight on 16 August 1948 at Muroc Army Air Field, the USAF changed its designation for fighter aircraft from “P” to “F”.”(1)
Source: (1) Wikipedia