Staff Writer (Updated: 5/4/2016):
The new aircraft arrived by way of the USAF's ever-evolving heavy-haul-minded programs of the 1960s that ultimately became the "Cargo Experimental - Heavy Logistics System" (CE-HLS). The service sought a four-engined product with a payload capability of 250,000lb flying over ranges of 3,000 miles without requiring aerial refueling. Because of the operating weights and power involved, it became obvious that a wholly-new fuel-efficient engine would be required. Additional qualities called for a fuselage with front and rear cargo access for "straight-through" loading / unloading.
The new proposal was drawn up in 1964 and some of the more traditional "big aircraft" American manufacturers responded like Boeing, Douglas, and Lockheed. These three passed into the design study phase and General Electric was commissioned to develop the engine. In the end, the Lockheed proposal won out over the others on cost despite authorities favoring the Boeing submission. The official announcement came in September of 1965 and the engines became the General Electric "TF39".
The Lockheed aircraft carried shoulder-mounted, swept-back wing mainplanes to which each held a pair of underslung engine nacelles towards their leading edges. The flightdeck was seated over a short, downward-sloped nose cone giving a commanding view of the ground ahead. The nosecone was hinged to open upwards allowing access to the hold within the body of the aircraft. The tail unit was raised aft of center mass to provide unrestricted access to the rear cargo area. The tail itself was arranged as a "T-style" assembly in which a single vertical fin supported very-high-mounted horizontal planes (these planes also swept-back). To round out the list of details, a multi-wheeled / multi-legged undercarriage configuration relying on new fewer than 28 wheels (!) was used to better balance the heavy aircraft on runways. The typical crew numbered seven to include three loadmasters and two flight engineers.
The cargo hold was designed to be large enough to accept a Sikorsky UH-60 type helicopter or similar up to the more modern Boeing V-22 Osprey systems. Beyond aircraft, the hold could also support all manner of cargo palettes and military vehicles giving the USAF a comprehensive heavy-lifter regardless of operating theater.
When introduced in 1970, the C-5 Galaxy became the largest aircraft in the world and remains one of the largest today (2016) - its dimensions include a wingspan of 222.8 feet, a length of 247 feet, and a height of 65 feet.