During the Battle of Guadalcanal, US Marine Corps forces found themselves facing Japanese forces dug in in fortified positions along the slopes of Mount Austen. The terrain made it extremely difficult to effectively use the available artillery (primarily M1A1 pack howitzers) heavily armed up-slope positions. 1Lt Preston Jamison was in the planning cell of the 1st Marine Division as the staff discussed options for neutralizing these threats. Jamison was the staff pilot assigned to General AA Vandegrift, CG of the 1st Marine Division. As they men considered options, Jamison thought back to his high school aeronautics class, and recalled page 23, where the text discussed on=pylon turns.
He remembered that, in a properly executed on-pylon turn, the lateral axis of the aircraft pointed constantly at the pylon. If he could mount a gun along the lateral axis of a plane, he felt sure he could aim the gun at a ground target by flying an on-pylon turn around it. After much discussion, General Vandegrift's approved the plan and assigned Capt TJ Smithers of the 164th Regiment to assist him in modifying Maj Gen Vandegrift's personal transport (a Beech JRB) for the operation.
Jamison and Smithers requisitioned an M1A1 75mm pack howitzer and proceeded to modify it for mounting in the JRB. First they removed the wheels, brakes and axle, then they cut off most of the trailing carriage portion. They then removed all interior equipment from the passenger compartment of the JRB, including the entry door. They then mounted the modified M1A1 in the doorway. The length of the 75mm shell dictated that room be left behind the gun to load, so a large portion of the gun extended outside the cabin. It was soon discovered that this caused the JRB to turn left unless significant right rudder was held - however, turning left was the main idea, anyway!
Jamison and Smithers also recognized the need for a way to check the aim without wasting a 75mm shell, so they mounted a .50-cabliber M2 machine gun to fire out of the circular window aft of the main gun.
The modified JRB went into action in on Jan 4, 1943. It was briefly an effective weapon against hardened positions in difficult terrain. However, the heavy gun took its toll on the airframe and after 7 missions, the JRB had to be scrapped. In the meantime, Beechcraft engineers in Wichita heard about the modification and were horrified. They declared the whole thing impossible and refused to cooperate with a program to build more. However, this experience may have influenced the design of the XA-38 Grizzly which had just been contracted for.
Because the JRB had been Maj Gen Vandegrift's personal transport, the Marines nicknamed the aircraft, "General VandeBoom."
