mm (.79 in) autocannon with 150 rounds. Clustering all the armament in the nose was unusual in U.S. aircraft, which typically used wing-mounted guns with trajectories set up to crisscross at one or more points in a convergence zone. Nose-mounted guns did not suffer from having their useful ranges limited by pattern convergence, meaning that good pilots could shoot much farther. A Lightning could reliably hit targets at any range up to , whereas the wing guns of other fighters were optimized for a specific range. The rate of fire was about 650 rounds per minute for the 20×110 mm cannon round (130-gram shell) at a muzzle velocity of about , and for the .50-caliber machine guns (43-gram rounds), about 850 rpm at velocity. Combined rate of fire was over 4,000 rpm with roughly every sixth projectile a 20 mm shell. The duration of sustained firing for the 20 mm cannon was about 14 seconds, while the .50-caliber machine guns worked for 35 seconds if each magazine were fully loaded with 500 rounds, or for 21 seconds if 300 rounds were loaded to save weight for long-distance flying. The Lockheed design incorporated tricycle undercarriage and a bubble canopy, and featured two turbosupercharged 12-cylinder Allison V-1710 engines fitted with counter-rotating propellers to eliminate the effect of engine torque, with the turbochargers positioned behind the engines, the exhaust side of the units exposed along the dorsal surfaces of the booms. Counter-rotation was achieved by the use of "handed" engines; the crankshafts of the engines turned in opposite directions, a relatively easy task for the V-1710 modular-design aircraft powerplant. The P-38 was the first American fighter to make extensive use of stainless steel and smooth, flush-riveted, butt-jointed aluminum skin panels. It was also the first military airplane to fly faster than in level flight. XP-38 and YP-38 prototypes Lockheed won the competition on 23 June 1937 with its Model 22 and was contracted to build a prototype XP-38 for US$163,000, though Lockheed's own costs on the prototype would add up to US$761,000. Construction began in July 1938 in an old bourbon distillery purchased by Lockheed to house expanding operations. This secure and remote site was later identified by Johnson as the first of five Lockheed Skunk Works locations. The XP-38 first flew on 27 January 1939 at the hands of Ben Kelsey. Kelsey then proposed a speed dash to Wright Field on 11