While working at Daimler Klemm had developed his ideas for a light aircraft, to be made of wood for strength and lightness. It should be easy to manufacture, aerodynamically efficient with low mass and wing loading, for which a low-powered engine would be sufficient. Klemm's first design, the Daimler L.15, was a light aircraft with a single 7.5 hp (5.5 kW) Indian motorcycle engine. This aircraft flew in early 1919 with a 12 hp (8.8 kW) Harley-Davidson engine.
Klemm then designed a squared-off version of the cylindrical fuselage of the L.15, which could be more easily built, designated the L.20, and founded his own company to produce it. This aircraft, of which more than 100 were built, was powered by a 20 hp (15 kW) Daimler engine designed by Ferdinand Porsche.
In 1928 Freiherr Friedrich-Karl von Koenig-Warthausen made a solo flight to Moscow in a Klemm L.20, then decided to keep on going, circumnavigating the world and earning himself the Hindenburg Cup, the highest German honour for aeronautical achievement.
From the L.20 was developed the Klemm L.25, later redesignated the Kl 25. Capable of being fitted with fourteen different engine types, over 600 were sold, and licenses to manufacture them were issued in Great Britain and the USA.
In January 26, 1930 Mohamed Sidki,the First Egyptian Eagle as he was called after he landed in Cairo, Egypt on his historical flight from Berlin to Cairo on a Klemm L25a. The Klemm L25a a small light plane was able to withstand ferocious Bora winds over the Mediterranean. As Sidki mentioned in a letter to Hans Klemm who had helped Sidki outfit his Klemm L25a for his long journey, "your beautiful little machine, the L25a has carried me through wind, snow and torrential rain without any damage to myself, the plane and the engine." Sidki became a political hero in Egypt by flying and landing a German made plane, defying British rule when he was denied landing rights on Egyptian soil.
Klemm suffered a setback in 1935, when the prototype Kl 35 crashed during testing at Rechlin. This was explained as a material defect, but was more likely due to overstressing of the wings. Klemm revised and brought the Kl 35 up to the required specifications and manufacturing began. Around 2,000 of the aircraft were built, both at Klemm's plant, and by Fieseler, and later by the Czech company Moravan Otrokovice, for the Luftwaffe as a trainer. During the war Klemm produced and designed a range of aircraft, such as the Klemm Kl 105, Kl 106 and Kl 107, as well as the Klemm Kl 151 and Kl 152.
The Klemm Aircraft Company was refounded in 1952 after the post-war ban on aircraft construction was lifted. Klemm himself by this time was semi-retired, and the company was run by his son Hanns-Jürgen Klemm. This companies first model - the Kl 107-A - made its maiden flight in mid-1956. In April 1959, Bölkow took over Klemm, and with the successor of the Kl 107-C named the Bölkow 207, the name of Klemm disappeared from the list of aircraft manufacturers.
Klemm aircraft included: