From May 1981 issue of "Airpower" magazine:

1. Mockup of the XP-89 in January, 1947, bore great resemblance to the XP-89
as built except for the small dihedral angle in the horizontal tail and the
four 20mm gun turret mounted in the nose. Nose turret was an Army Air Force
requirement specified in Military Characteristics of Aircraft (All-Weather),
23 November 1945. Provisions for a nose turret were also included in the
rival Curtiss XP-87. In background is experimental X-4 Skylancer research
plane.

2. Separate nose turret mockup photographed in August 1946. The gun cover is
removed showing the gun carriage centered. The four 20mm weapons were
mounted on a central structural carriage with a radar fire control system
centered amid the guns inside the ball portion of the turret. Breech end of
the gun carriage was mechanically moved in a Y-Z plane within the nose
structure to provide a 30° cone of fire. Ball end of the gun carriage rode
In a socket-like portion of the forward structure of the nose assembly.

3. The XP-89 mockup showing the proposed four 20mm gun turret installation
including the ammo feed chutes and boxes on the right hand side of the
photograph. This turret concept was an indigenous Northrop design and should
not be confused with the Martin turret installed in a later F-89A. The
turret shown was never produced. Instead, four fixed 20mm guns were
specified as an interim installation for the XP-89 flight article.

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