From the 456th FIS website. "While the P-38 design was starting to prove itself in testing and combat, one underlying problem still existed. Compressibility was still a major obstacle in the performance of the P-38, and it led to many rumors and myth about the flight characteristics. One prototype P-38 was modified for dive testing. An older P-38 E was the aircraft used for this experimentation. The forward gondola section was moved forward by thirty inches, and the cockpit was move forward by thirty-six inches. The cockpit flight controls were the same, but there was room behind the pilot for a co-pilot/observer/test engineer. This modified P-38 E "Swordfish" first too flight on June 2, 1943 and was immediately put into extensive dive tests. The tests would have limited results, but this design led into another use for this type of P-38.
AAF training before Pearl Harbor was only designed to train between 12,000 - 13,000 pilots each year. After the attack, plans were changed and the numbers were increased significantly. However, the problem was that the government was unprepared for any armed conflicts, and pilots were being forced into the P-38 without adequate training. There were no twin-engine trainers available, but when the British balked at purchasing the Model 322 Lightning I fighters, they were employed as trainers. They were basically stripped down models of the P-38, but they were still rather advanced for the average trained pilot taking the controls for the first time. Johnson envisioned the P-38 Swordfish to be used to take pilot trainees along for demonstration flights with an experienced pilot at the controls. This would dispel many rumors and raise confidence in inexperienced pilots. Lockheed test pilot Jimmy Mattern was sent on tour with a Swordfish version of the P-38. He performed maneuvers that were normally feared by the recruits, such as rolling into a "dead" engine and many low-level aerobatics. After five months of training sessions, P-38 accident rates dropped from 6.5% to 1.5%. Mattern would receive the Civil Medal of Merit for his actions, which saved the lives of many recruits. Once again, the P-38 demonstrated its versatility and value to the Allied cause."

