The USAAC notified Kartveli that the XP-47A and the XP-44 Rocket contracts were canceled, since the P-43/XP-44 airframe was too small to meet the new requirements. (The XP-44 Rocket was based on the P-43 Lancer airframe with an even more powerful radial engine and never made it past the mock-up stage.) Kartveli then took the Seversky AP-4, a daring concept with the exhaust-driven turbocharger in the rear fuselage belly, and quickly prepared a rough sketch of a new XP-47B prototype. He planned to use the new 2,000 hp Pratt & Whitney Double Wasp XR-2800-21, eighteen cylinder, two-row radial engine, which was the largest and most powerful aircraft engine ever developed in the United States. The new design would incorporate eight 0.50 inch machine guns, additional ammunition, increased fuel capacity, and armor protection for the pilot. (The final fuel load was slightly under the capacity required, but this was overlooked as the aircraft met performance specifications.) Additionally, the airplane would include an efficient super-charging duct system that would offer the least interrupted airflow. Kartveli therefore adopted the unorthodox method of designing this feature first, and then building up the fuselage around it. Despite the fact that the supercharger was in the tail and the engine was in the nose, the arrangement worked quite well, providing a system that was durable and less susceptible to battle damage.