The Lockheed C-5 Galaxy is a large military transport aircraft designed and built by Lockheed, and now maintained and upgraded by its successor, Lockheed Martin. It provides the United States Air Force (USAF) with a heavy intercontinental-range strategic airlift capability, one that can carry outsized and oversized loads, including all air-certifiable cargo. The Galaxy has many similarities to the smaller Lockheed C-141 Starlifter and the later Boeing C-17 Globemaster III. The C-5 is among the largest military aircraft in the world. All 52 in-service aircraft have been upgraded to the C-5M Super Galaxy with new engines and modernized avionics designed to extend its service life to 2040 and beyond.

The C-5 Galaxy's development was complicated, including significant cost overruns, and Lockheed suffered significant financial difficulties. Shortly after entering service, cracks in the wings of many aircraft were discovered and the C-5 fleet was initially restricted in capability until corrective work was completed.

The USAF has operated the C-5 since 1969. In that time, the airlifter supported US military operations in all major conflicts including Vietnam, Iraq, Yugoslavia, and Afghanistan, as well as allied support, such as Israel during the Yom Kippur War and operations in the Gulf War. The Galaxy has also distributed humanitarian aid, provided disaster relief, and supported the US space program.

In 1961, several aircraft companies began studying heavy jet transport designs that would replace the Douglas C-133 Cargomaster and complement Lockheed C-141 Starlifters. In addition to higher overall performance, the United States Army wanted a transport aircraft with a larger cargo bay than the C-141, whose interior was too small to carry a variety of their outsized equipment. This need led to the CX-4 requirement of July 1962, for which Lockheed, Boeing, Convair, and Douglas proposed six-engined designs. When the US Army judged the CX-4 specification inadequate for its requirements, by late 1963 the CX-4 specification gave way to the CX-HLC requirement specified an airlifter with four engines, an equipped gross weight of 550,000 pounds (249,000 kg), a maximum payload of 180,000 lb (81,600 kg), and a speed of Mach 0.75 (500 mph or 805 km/h). The cargo compartment was 17.2 ft (5.24 m) wide by 13.5 feet (4.11 m) high and 100 ft (30.5 m) long with front and rear access doors. USAF studies showed that high-bypass turbofan engines were needed for thrust and fuel efficiency requirements.

We started to build the C-5 and wanted to build the biggest thing we could … Quite frankly, the C-5 program was a great contribution to commercial aviation. We'll never get credit for it, but we incentivized that industry by developing [the TF39] engine.

The criteria were finalized and an official request for proposal was issued in April 1964 for the "Heavy Logistics System" (CX-HLS) (previously CX-HLC, "Heavy Logistics Capability"). In May 1964, proposals for aircraft were received from Boeing, Douglas, General Dynamics, Lockheed, and Martin Marietta. General Electric, Curtiss-Wright, and Pratt & Whitney submitted proposals for the engines. After a downselect, Boeing, Douglas, and Lockheed were given one-year study contracts for the airframe, along with General Electric and Pratt & Whitney for the engines. All three of the designs shared a number of features. The cockpit was placed well above the cargo area to allow for cargo loading through a nose door. The Boeing and Douglas designs used a pod on the top of the fuselage containing the cockpit, while the Lockheed design extended the cockpit profile down the length of the fuselage, giving it an egg-shaped cross section. All of the designs had swept wings, as well as front and rear cargo doors, allowing simultaneous loading and unloading. Lockheed's design featured a T-tail, while the designs by Boeing and Douglas had conventional tails.

The Air Force considered Boeing's design to be better than that of Lockheed, but Lockheed's proposal was the lowest total-cost bid. Lockheed was selected as the winner in September 1965, then awarded a contract in December 1965. General Electric's TF39 engine was selected in August 1965 to power the new transport plane. At the time, GE's engine concept was revolutionary, as all engines before had a bypass ratio less than two-to-one, while the TF39 promised and would achieve a ratio of eight-to-one, which had the benefits of increased engine thrust and lower fuel consumption. Boeing lost the military contract, but went on to develop its proposal into the successful civilian Boeing 747, the world's first wide-body airliner, with 1,574 aircraft built when manufacturing ended in 2022 after 54 years of production.

The first C-5A Galaxy (serial number 66-8303) was rolled out of the manufacturing plant in Marietta, Georgia, on 2 March 1968. On 30 June 1968, flight testing of the C-5A began with the first flight, flown by Leo Sullivan, with the call sign "eight-three-oh-three heavy". Flight tests revealed that the aircraft exhibited a higher drag divergence Mach number than predicted by wind tunnel data. The maximum lift coefficient measured in flight with the flaps deflected 40° was higher than predicted (2.60 vs. 2.38), but was lower than predicted with the flaps deflected 25° (2.31 vs. 2.38) and with the flaps retracted (1.45 vs. 1.52).