The Antonov An-225 was designed to airlift the Energia rocket's boosters and the Buran space shuttle for the Soviet space program. It was developed as a replacement for the Myasishchev VM-T. The An-225's original mission and objectives are almost identical to that of the United States' Shuttle Carrier Aircraft.
The An-225 first flew on 21 December 1988 with a 74-minute flight from Kiev. The aircraft was on static display at the Paris Air Show in 1989 and it flew during the public days at the Farnborough air show in 1990. Two aircraft were ordered, but only one An-225 (registration CCCP-82060 later UR-82060 was finished. It can carry ultra-heavy and oversize freight, up to 250,000 kg (550,000 lb) internally, or 200,000 kg (440,000 lb) on the upper fuselage. Cargo on the upper fuselage can be 70 metres (230 ft) long.
A second An-225 was partially built during the late 1980s for the Soviet space program. The second An-225 included a rear cargo door and a redesigned tail with a single vertical stabilizer. It was planned to be more effective for cargo transportation. Following the collapse of the Soviet Union in 1991 and the cancellation of the Buran space program, the lone operational An-225 was placed in storage in 1994. The six Ivchenko-Progress engines were removed for use on An-124s, and the second uncompleted An-225 airframe was also stored. The first An-225 was later re-engined and put into service.
By 2000, the need for additional An-225 capacity had become apparent, so the decision was made in September 2006 to complete the second An-225. The second airframe was scheduled for completion around 2008, then delayed. By August 2009, the aircraft had not been completed and work had been abandoned. In May 2011, the Antonov CEO is reported to have said that the completion of a second An-225 Mriya transport aircraft with a carrying capacity of 250 tons requires at least $300 million, but if the financing is provided, its completion could be achieved in three years. According to different sources, the second jet is 60–70% complete.
In April 2013, the Russian government announced plans to revive Soviet-era air launch projects that would use a purpose-built modification to the An-225 as a midair launchpad.
Based on Antonov's earlier An-124, the An-225 has fuselage barrel extensions added fore and aft of the wings. The wings also received root extensions to increase span. Two more Progress D-18T turbofan engines were added to the new wing roots, bringing the total to six. An increased-capacity landing gear system with 32 wheels was designed, some of which are steerable, enabling the aircraft to turn within a 60 m (200 ft) wide runway. Like its An-124 predecessor, the An-225 has nosegear designed to kneel so cargo can be more easily loaded and unloaded. The An-124’s rear cargo door and ramp were removed to save weight and the empennage was changed from a single vertical stabilizer to a twin tail with an oversized horizontal stabilizer. The twin tail was essential to enable the plane to carry large, heavy external loads that would disturb the airflow around a conventional tail. Unlike the An-124, the An-225 was not intended for tactical airlifting and is not designed for short-field operation.
Initially the An-225 had a maximum gross weight of 600 t (1,300,000 lb), but from 2000 to 2001 the aircraft underwent modifications such as the addition of a reinforced floor, which increased the maximum gross weight to 640 t (1,410,000 lb) at a cost of US$20M.
Both the earlier and later takeoff weights establish the An-225 as the world's heaviest aircraft, being heavier than the double-deck Airbus A380. It is surpassed in other size-related categories, however: Airbus claims to have improved upon the An-225's maximum landing weight by landing an A380 at 591.7 tonnes (1,304,000 lb) during tests, and the Hughes H-4 Hercules, known as the "Spruce Goose", had a greater wingspan and a greater overall height. But the Spruce Goose was 20% shorter and overall lighter, due to the materials used in its construction. It also flew only once, making the An-225 the largest aircraft in the world to fly multiple times.
The An-225's pressurized cargo hold is 1,300 m3 (46,000 cu ft) in volume; 6.4 m (21 ft) wide, 4.4 m (14 ft) high, and 43.35 m (142.2 ft) long — longer than the first flight of the Wright Flyer. The Boeing Dreamlifter has a bigger cargo hold at 1,840 m3 (65,000 cu ft).