The original Boeing 737-100 and -200 used the long and narrow Pratt & Whitney JT8D engine, which fit entirely under the wing and enabled a relatively low ground clearance. In the late 1960s, when the aircraft was introduced, its low-to-the-ground design was beneficial, allowing easier boarding from optional on-board airstairs and hand-loading of baggage into holds. But as airports were modernized with jet bridges and mechanized baggage handling, the design was less necessary, and the low ground clearance would prove to be a design constraint for Boeing engineers.[10]