The Horten H.IX, RLM designation Ho 229 (or Gotha Go 229 for extensive re-design work done by Gotha to prepare the aircraft for mass production) was a German prototype fighter/bomber designed by Reimar and Walter Horten to be built by Gothaer Waggonfabrik. Developed at a late stage of the Second World War, it was one of the earliest flying wing aircraft to be powered by jet engines.

The Ho 229 was designed in response to a call made in 1943 by Hermann Göring, the head of the Luftwaffe, for light bombers capable of meeting the "3×1000" requirement; namely, to carry 1,000 kilograms (2,200 lb) of bombs a distance of 1,000 kilometres (620 mi) with a speed of 1,000 kilometres per hour (620 mph). Only jet propulsion could achieve the required speed, but such engines were very fuel-hungry, necessitating considerable effort across the rest of the design to meet the range requirement. The flying wing configuration was favoured by the Horten brothers due to its high aerodynamic efficiency, as demonstrated by their Horten H.IV glider. In order to minimise drag, the Ho 229 was not fitted with extraneous flight control surfaces. Its ceiling was 15,000 metres (49,000 ft). The Ho 229 was the only design that came close to the requirements, and the Horten brothers quickly received an order for three prototypes after the project gained Göring's approval.

Due to the Horten brothers' lack of suitable production facilities, Ho 229 manufacturing was contracted out to Gothaer Waggonfabrik; however, the company allegedly undermined the project by seeking the favour of Luftwaffe officials for its own flying wing design.^{[citation needed]} On 1 March 1944 the first prototype H.IX V1, an unpowered glider, made its maiden flight, followed by the H.IX V2, powered by Junkers Jumo 004 turbojet engines in December 1944. However, on 18 February 1945 the V2 was destroyed in a crash, killing its test pilot. Despite as many as 100 production aircraft being on order, none were completed. The nearly complete H.IX V3 prototype was captured by the American military and shipped to the United States under Operation Paperclip. It was evaluated by both British and American researchers before entering long term storage. The H.IX V3 is on static display in the Smithsonian National Air and Space Museum.

During the early 1930s the Horten brothers had become interested in the flying wing configuration as a method of improving the performance of gliders. At that time the German government was actively funding glider clubs as a response to the production of aircraft suitable for military roles being forbidden by the Treaty of Versailles. The flying wing layout theoretically offered the lowest possible weight, and without the added drag of the fuselage. Their first aircraft of this configuration was the Horten H.IV.

In 1943 Hermann Göring issued a request for design proposals for a bomber capable of carrying a 1,000 kilograms (2,200 lb) load over 1,000 kilometres (620 mi) at 1,000 kilometres per hour (620 mph) which was known as the "3×1000 project". German bombers could reach Allied targets across Great Britain, but were suffering devastating losses from Allied fighters. At the time, no conventional means for aircraft designers to meet these goals seemed viable because while the new Junkers Jumo 004B turbojets provided the speed, excessive fuel consumption limited range. The Horten brothers concluded that a low-drag flying wing design could meet the goals, as by reducing drag, cruise power could be reduced so the range requirement could then be met. They put forward their private project, the H.IX, as the basis for the bomber.

While removing the vertical stabilizer reduced drag, it caused yaw control problems. In traditional aircraft, a vertical stabilizer works passively to ensure that sideslip is minimized by producing a force perpendicular to itself when any sideslip occurs. The lack of a vertical stabilizer meant that flying without any active yaw control would lead to an uncontrolled sideslip, and potentially flat spins. This was resolved with split ailerons, which increase drag on one side. While designs without vertical stabilizers require more active control by the pilot or a future flight control systems and lead to bank angle restrictions, they do reduce aerodynamic drag slightly.^{[failed verification]}

The Government Air Ministry (Reichsluftfahrtministerium) quickly approved Horten's proposal, but ordered the addition of two 30 mm (1.2 in) cannons, as they felt the aircraft might also be useful as a fighter due to it being significantly faster than existing Allied aircraft. German officials assigned the designation Ho 229 to the aircraft. Göring was reportedly impressed with the design and personally intervened to ensure that three prototypes were ordered at a cost of 500,000 Reichsmarks. The Air Ministry issued an order for 100 production aircraft, but this was later reduced to 20. Furthermore, as the Horten brothers lacked production facilities, it was decided that the manufacturing would be done by an established company, Gothaer Waggonfabrik. This arrangement was complicated by Gothaer's alleged efforts to persuade the authorities to favour its own projects, which included flying wings, over the Ho 229.

Observing the Ho 229's design and development difficulties, Russell Lee, the chair of the Aeronautics Department at the National Air and Space Museum, speculated that an important motivation for the Horten brothers was to prevent them and their workers from being assigned dangerous roles by the German military. Looking beyond the Ho 229, the Horten brothers produced numerous flying wing designs, such as the Horten H.VII fighter-trainer and the Horten H.XVIII Amerikabomber. According to the aviation historian Jean-Denis G.G. LePage, other German wartime projects were inspired by the Horten brother's work.