In the lead-up to World War II, the Germans made successive improvements to their military Enigma machines. By January 1939, additional rotors had been introduced so that three rotors were chosen from a set of five (hence there were now 60 possible wheel orders) for the army and air force Enigmas, and three out of eight (making 336 possible wheel orders) for the navy machines. In addition, ten leads were used on the plugboard, leaving only six letters unsteckered. This meant that the air force and army Enigmas could be set up in 1.5×1019 ways. In 1941 the German navy introduced a version of Enigma with a rotatable reflector (the M4 or Four-rotor Enigma) for communicating with its U-boats. This could be set up in 1.8×1020 different ways.