Your best bet is to go to this link
and then click on "long description." Below is an excerpt.
Reimar began to think seriously about the jet wing at the end of 1940. Fiercely independent and lacking the proper intellectual credentials, Reimar worked outside the mainstream German aeronautical community whenever he could. The authorities denied him access to wind tunnels to test his ideas, in part because of Reimar's youth and lack of advanced education, so he developed his designs using flying models and piloted aircraft. He had already successfully flown more than 20 aircraft by 1941 but a jet-propelled wing would be heavier and faster than any previous wing. To minimize the risk of experimenting with such an advanced aircraft, Reimar built and tested several interim designs, each one moderately faster, heavier, or more advanced in some significant way than the one before it. . . .
At the beginning of 1943, Walter heard Göring complain that Germany was fielding 17 different types of twin-engine military airplanes with similar, often mediocre, performance, but spare parts were not interchangeable between any two of these designs. He decreed that henceforth he would not approve for production another new twin-engine airplane unless it could carry 1,000 kg (2,210 lb) of bombs to a 'penetration depth' of 1,000 km (620 miles, penetration depth defined as 1/3 the range) at a speed of 1,000 km/h (620 mph). Asked to comment, Reimar announced that only a warplane equipped with jet engines had a chance to meet those requirements.
In August Reimar submitted a short proposal for an all-wing aircraft that came close to achieving Göring's specifications, who then issued the brothers a contract, and demanded the new aircraft fly in 3 months! Reimar responded that the first Horten IX prototype could fly in six months and Göring accepted this schedule after revealing his desperation to get the new fighter in the air with all possible speed.