The preceding Yorktown-class carriers formed the basis from which the Essex class was developed. Designed to carry a larger air group, and unencumbered by pre-war naval treaty limits, the USS Essex (CV-9) was over sixty feet longer, nearly ten feet wider in beam and more than a third heavier. A longer, wider flight deck and a deck-edge elevator facilitated more efficient aviation operations, enhancing the ship's offensive and defensive air power. Machinery arrangement and armor protection was greatly improved from previous designs. These features, plus the provision of more anti-aircraft guns, gave the ships much enhanced survivability. In fact, none of the Essex-class carriers were lost and two of them, Franklin (CV-13) and Bunker Hill (CV-17), came home under their own power even after receiving heavy damage.