Flettner's initial Fl 184 design was successful, so the inventor added power to the main rotor. He also developed a collective pitch control, which allowed the pilot to increase the pitch of all of the lifting blades simultaneously. Flettner also removed the autogyro's main propeller and replaced it with two smaller propellers on outriggers located on the side.

In 1937, while the Focke-Achgelis Fa 61 was making its impressive test flights, Flettner began developing the Fl 265. This was a small helicopter, compared to its only contemporary, the Focke-Achgelis Fa 223. The Fl 265, which was called a synchropter because of its rotor configuration, had two counter-rotating rotors set close together and splayed outwards. The rotors intermeshed like the blades of an eggbeater. Flettner received a small production contract from the German Navy in 1938, and the aircraft made its first flight in May 1939. The aircraft proved impressively controllable in flight and was a major improvement over the Focke-Achgelis designs.

In 1940, Flettner debuted an improved version designated the Fl 282 Kolibri (hummingbird). The Kolibri could fly at almost 90 miles per hour (145 kilometres per hour), could reach 13,000 feet (3,962 meters), and could carry an 800-pound (383-kilogram) load. The fuselage was approximately 21.5 feet (6.5 meters) long, and the craft was 7.2 feet (2.1 meters) high. The rotor diameter was approximately 39.2 feet (12 meters) and had the same synchropter configuration as the earlier Fl 265.

Flettner designed his craft to carry two people, a pilot and an observer. The pilot sat in front of the rotors in an open cockpit affording good visibility. The observer sat in a single compartment behind the rotors, facing the rear. The observer could spot submarines at sea or troop movements on the battlefield. The Kolibri was one of the first helicopters designed with a clear military mission.

The German Navy (Kriegsmarine) was impressed with the Kolibri and wanted to evaluate its use for submarine spotting. However, critics argued that fighter planes would easily attack the slow-flying craft. In 1941, the Navy conducted an evaluation using two fighter planes to stage a mock attack on a Fl 265. The fighters could not hold the agile craft in their gun-sights.

Flettner also demonstrated that the little craft could land on a ship, even in heavy seas. Naval leaders were impressed and, in 1940, ordered several dozen of the craft with the clear intention of mass-producing them. Allied bombing ended production efforts, but 24 of the aircraft still entered service, with a number of them being used for escort service, flying off the gun turrets of ships to spot submarines, and performing re-supply missions even in poor weather conditions. The German Army also evaluated this type of aircraft.

The Fl 282 served in the Baltic, North Aegean, and Mediterranean Seas. Only three of the craft survived the war; the Germans destroyed the rest to prevent them from falling into Allied hands. Two of the survivors went to the United States and Britain, the third to the Soviet Union.

The Fl 282 was designed so the rotor blades and landing gear could be removed and the helicopter stored in a compact area such as the pressure tank of a U-boat. There is no evidence that it was ever used this way. It was intended to search for submarines, and in the fairly clear waters of the Mediterranean, a pilot could see a submerged submarine as deep as 130 feet (40 meters). He could match the speed and course of the submarine and radio the position to the convoy. The pilot could also mark the sub's position with a smoke bomb. But the helicopter was too small to carry weapons, although some tests were conducted with small anti-submarine bombs. There is no good information on the helicopter's actual use during the war.

The Kolibri's intermeshing rotors represented the fourth approach to solving the control and torque problems, after Bréguet's stacked coaxial counter-rotating blades, Focke's widely spaced counter-rotating blades, and Sikorsky's tail rotor. The Kaman Huskie, which saw U.S. Air Force, Navy, and Marine service during the 1950s and 1960s, and the Kaman K-Max single-seat aerial crane used this design in the 1990s. The design has not proven to be long lasting or popular. The biggest problems with this design are that the helicopters are slow compared to other types and the rotors endanger people on the ground.