How did the P-38 make its two propellers spin in opposite directions?

Lockheed's design mounted two 12-cylinder Allison V-1710 engines of 1,000 hp (750 kW), turbocharger-supercharged, with counter-rotating propellers to eliminate the effect of engine torque. The counter-rotation was achieved with "handed" engines: the crankshafts of the two engines turned in opposite directions. With the V-1710, a modular-design engine, this variant was a relatively easy task. The turbochargers sat behind the engines, with the exhaust side visible above the booms.

In the prototype, the blades turned inward, toward the cockpit, at the top of their arc. In the YP-38 pre-production aircraft, fitted with Allison V-1710-27 and -29 engines and B-2 turbochargers, the direction of rotation was reversed: the blades began turning outward, away from the cockpit, at the top of their arc. This outward rotation of the counter-rotating propellers was a distinctive feature of the P-38's design, and also the source of one of its own problems.

Not all Lightnings had counter-rotation. The overseas allies wanted their Allison engines to be fully common with those of the numerous Curtiss P-40 Tomahawks they had ordered, so they requested the aircraft with two engines that both turned to the right, without counter-rotation, and without turbochargers. These were the aircraft to which the British, who took over the entire order after the fall of France in June 1940, gave the service name "Lightning".