The Antonov An-225 was the largest aircraft in the world and came with six engines. The United States designed the Boeing B-52 Stratofortress in the 1950s with a whopping eight engines. But the 1950s were a time when jet engines were still in their infancy and were underpowered and unreliable. By the 1960s, Boeing and Lockheed designed the Boeing 747 and C-5 Galaxy, respectively, with just four engines. Now, all subsonic passenger jets are designed with just two engines.