In reality, only one experimental vertical takeoff interceptor was available in 1944. This simple innovative project designated Bachem Projekt 20, which was accepted by the Air Ministry and received the official RLM designation Ba 349, was code-named Natter (Viper). It was manufactured in small numbers prior to the end of the war. A second design with vertical takeoff capability, the He P 1077 (see p. 152), failed to materialize except for a few gliders that may have been completed prior to the war's conclusion.

The Bachem Werke GmbH was founded on February 10, 1942, by Dipl.-Ing. Erich Bachem, formerly the Technischer Direktor of the Fieseler firm. The company manufactured spare parts for piston-engine fighters and other aircraft equipment before the Natter project was created.

The BP 20 was projected as a small lightweight expendable interceptor, capable of destroying any enemy bomber using the least possible weapon expenditure. To achive this objective, this ambitious project employed a vertical rocket-assisted takeoff followed by separate descent and landing of pilot and aircraft by separate parachutes. It was believed that pilots having little or no experience would need only rudimentary flight and gunnery instruction, rather than spending valuable training resources on the finer points of flight training. Erich Bachem reasoned that, a reasonable number of such interceptors and launch sites could be installed around key industrial targets, to make attacking Allied bombers pay a prohibitively high price. Other attributes of Natter included savings in the amount of steel and aviation fuel and the ability to be quickly transported from small, camouflaged sites. The ability to recover the rocket motor for reuse was considered an important feature of this aircraft, which was essentially a manned missile.. There were even plans to launch the interceptor from ships if the need arose.

The BP 20 was of wood construction and was to be built without the use of gluing presses. Most of the parts could be made in small woodworking shops through Germany, without interfering with the existing needs of the aircraft industry. According to Erich Bachem, ) only 600 man-hours were required for the production of one airframe, excluding the HWK 509 A-2 rocket motor, which was a relatively simple to manufacture when compared to sophisticated turbojet.

The fuel capacity was to consist of 119 US gallons (450 liters) T-Stoff and 66 US gallons (250 liters) C-Stoff, carried in separate tanks. The available fuel was sufficient for 80 seconds at full power, developing a thrust of about 3,750 lb (1,700 kg). Takeoff assistance was provided by four solid-fuel rockets SR 34 which produced an additional thrust of 2,200 lb (1,000 kg) for twelve seconds.

Natter's weapon systems were simple and potentialy devastating. They comprised either a honeycomb 24 electrically fired 73 mm F6hn air-to-air rockets, or 32 R40 air-to-air missiles located behind a jettisonable cover in nose section. The alternative, the Rheinmetall SG 119 consisted of six clusters, each cluster containing seven MK108 barrels grouped together in a cylinder with the clusters arranged about the Viper's nose as in a revolver.

Bachem submitted his Natter project to the OKL and simultaneously to the SS-F6hrungshaptamt (SS Planning Office), in August 1944. One month later, a contract for fifteen experimental BP 20 aircraft was awarded, and a few weeks later, Natter was included in the J5gerNotprogramm (Emergency Fighter Program). The first experimental aircraft, the Ba 349 M1 and M2, were under construction in October 1944, at a time when the RLM believed the Natter could be successfully employed against Allied heavy bombers, including the anticipated American Boeing B-29.30 The first batch of fifty Ba 349s was ordered for delivery between October 1944, and January 1945. A measure of the importance attached to the program is in the substantial order of 200 Vipers at the start of mass production.

The installation of parachutes delayed flight testing of the first target defense prototype, the Ba 349 M1, until November 1944. The Ba 349 M2 was completed soon thereafter and the first takeoff under air-tow of the third prototype, Ba 349 M3, was made at Neuburg on the Danube on December 14, 1944. A second flight behind a He 111 by the DFS, followed eight days later. After successful completion of ground tests near Bad Waldsee on December 18, 1944, the first vertical launch from a ramp was scheduled. This event was marred when the Viper caught fire as a result of a technical fault. The next attempt was made four days later at Heuberg Hill near Stetten am kalten Markt. The aircraft was towed to an altitude of 2,460 ft (750 m) and parachutes were deployed to carry the Viper and its simulated pilot safely to ground. The second takeoff occurred on December 29, 1944, without serious incident while simultaneously, other Vipers were towed into the air for further testing.

French armor advanced into Waldsee on April 1945 and a great number of spare parts was captured- Only a few days before the French arrived, fifteen rocket engines destined for Vipers had been thrown into Lake Waldsee to prevent their capture. The secret was not well kept however and all were later recovered.

Plans for mass production of the Ba 349 A-1 were authorized on March 1, 1945, but only a few Natters were actually completed. These were followed by the improved Ba 349 B-1 (Entwurf 2) interceptors whkb were to be produced at Waldsee, but few were actually completed.

On of the models were powered by a solid-fuel rocket to evaluate takeoff characteristics. Practical tests carried out at Peenemunde, where a first test conducted during February 1945, proved unsuccessful. Willy A. Fiedler, a testing engineer working for the RLM, was sent to the Heuberg Hills to oversee the program Erich Bachem is quoted after the war as having said that about twenty Vipers had been used for practical tests. Fifteen were of the A-series, and four B-series aircraft. All were constructed at Waldsee. Still others were assembled by the Wolf Hirth glider factory. Four additional Ba 349s, possibly of the B-series, were captured at the end of the war by Allied forces near St. Leonhard, Austria.

Some views of the Bachem 349 Natter (Viper) cockpit (see Dave Caldwell's paper model!)

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