The Horten H.XVIII existed as a proposed jet-powered bomber for the German Luftwaffe during World War 2. The bomber was part of the "Amerika Bomber" initiative calling for a transatlantic bombing platform able to strike targets in far-off places - namely the United States whose homefront was virtually out of reach from current generation German bombers yet causing so much disruption to the German war effort in Europe. The H.VIII was another of the Horten Brother's "flying wing" proposals that utilized a blended body-wing surface while lacking vertical appendages and promoting excellent internal storage space to be taken up by fuel and drop ordnance. The H.XVIII carried many of the design features established in the Horten Brother's most memorable design, the Ho 229 flying wing, which was under development by the end of the war.
While the Germans were some distance away from developing their own atomic bomb, it is conceivable that an Amerika Bomber would have been used to deliver such ordnance over American cities. Luftwaffe head Hermann Goring himself believed this as an achievable goal. Fears of a German atomic bomb spurred the Americans to develop the weapon first as well as the aircraft to deliver it - the Boeing B-29 "Superfortress".
It was in late 1944 that the German Air Ministry was considering a long range bomber to disrupt American involvement in the war by delivering bombs to American cities and factories by way of the Atlantic route. Major German aviation defense players were all considered but none were able to meet the Luftwaffe demand concerning range - 6,835 miles being the bare minimum proposed by Goring. The Horten Brothers were approached with the requirement and accepted the challenge to design such a bomber utilizing their interesting flying wing concept that originally began with unpowered glider tests some time earlier. This led to the Brothers evolving their existing Ho 229 product by simply enlarging its dimensions and adding additional engines (to six from the original two) and the project gained the designation of "H.XVIII" (P.18) though this was a reportedly unofficial marker.
The engine of choice was to become either the BMW 003 or Junkers Jumo 004 turbojet arranged in two banks of three engines (for a total of six seated side-by-side-by-side). The engines would take up a place in the rear fuselage, aspirated through six portholes at the fuselage's leading edge and exhausting over the fuselage's trailing edge promoting a smooth overall contour to the aircraft's shape. Estimate performance figures included a maximum speed of 560 miles per hour with a range of 7,460 miles - though none of these values were ever proven before the end.
The overhead appearance of the aircraft was of a slightly flattened triangle with the cockpit seated at the apex housing a crew of three under a "greenhouse-style" canopy assembly providing good views of the upcoming terrain. The wings feature sweep back along their leading and trailing edges and were rounded at their tips. Because the H.XVIII lacked any vertical surfaces, controlling was to be handled through a series of flaps built into the wings - namely at the trailing edges. Construction of the aircraft was set to include a steel tube framework with metal skinning at the fuselage and a wood/glue based combination for the wings. As it stood, the H.XVIII was to have been a massive aircraft with a wingspan reaching out to 138 feet.
Proposed defense was a single turret manned by a dedicated gunner. This emplacement was armed with 2 x 30mm Mk 108 series autocannons to defend the aircraft from interception from trailing aircraft. The turret was situated directly aft of the cockpit to keep the crew together and improve communications between the members. Additional defense was through a similar turret mounted directly beneath the dorsal fitting - armed with the same weaponry and intended for defense of the aircraft's vulnerable underside. The undercarriage was jettisonable to simply construction and operation with landing by way of a skid system (similar in concept to the rocket-powered Messerschmitt Me 163 "Komet" interceptor).
In a February 1945 meeting, held at a time when the German war situation was decidedly desperate, the H.XVIII was selected ahead of competing long range, heavy bomber designs. The product was designated as the "H.XVIIIA" and engineers from both Messerschmitt and Junkers were involved to speed development. These engineers agreed that the Horten design could benefit from a more traditional, dedicated vertical tail fin fitted on the rear section of the fuselage to help improve directional controlling. The design decision was not well-accepted by the Brothers who took an evolved design, dimensionally similar though promising even greater operational ranges by way of 4 x Heinkel HeS 011 turbojets, to Hermann Goring himself. Goring was sufficiently impressed to order this newer design for construction as the "H.XVIIIB". The A-model proposal eventually fell to history with no real physical work having been completed before the end of the war.
The H.XVIIIB was a lighter overall design than the A-model before it and given a traditional wheeled undercarriage system that was not jettisonable. It also remained fixed in flight while faired over by an assembly that also was to house one of the HeS 011 turbojets each. Interestingly, the aircraft was to lack a nose landing gear leg and be supported through its multi-wheel main leg structures - a simplification for the manufacture process to an extent by doing away with a more complicated retracting undercarriage approach. Again, the multi-person crew (three) was to be held at the apex of the triangle design though now under a cleaner "bubble-style" canopy promising excellent views. Defensive armament was entirely optional as the operating altitude of the bomber would neutralize interception by Allied fighters. At any rate, the Hortens proposed 2 x 30mm MK 108 autocannons under the cockpit to satisfy any concerned authorities. Performance estimates included a maximum speed of 530 miles per hour and a range out to 6,835 miles while flying at altitudes of 52,500 feet. The end of the war in Europe in May of 1945 ended all work on the B-model - of which there was little tangible progress made as construction was not slated to begin until the fall of that year.
In one final revision - this one reportedly pushed by Messerschmitt and Junkers engineers - the H.XVIIIB H.XVIIIB-2 (also known as H.XVIIIC) was to feature 6 x BMW 003 turbojet engines under the wings and a dorsally-mounted turret armed with 2 x 20mm MK 151 cannons. Like all the other H.XVIII submissions, this too fell to naught.
While nothing became of the wartime H.XVIII program, Reimar Horten reestablished his work in Argentina following the end of the war. He managed to further his flying wing concept through the two-man DINFIA IA 38 prototype of which one was ultimately built and this first flown in December of 1960. The design proved to showcased poor performance because of its engines (4 x radial piston types of 450 horsepower each) and was ultimately abandoned, the hulk lost to a suspicious fire before being scrapped.
Work on flying wing concepts continued during the Cold War years, particularly in the United States by Northrop which its founder, Jack Northrop, had chased flying wing designs for decades - even in the prewar years as the flying concept was not solely a German venture. This eventually produced the Northrop YB-49, forerunner to today's highly-advanced B-2 "stealth bomber" (both detailed elsewhere on this site).