Even as early as 1943 it was becoming clear that Messerschmitt's super-interceptor, the Me 163, was going to be difficult to use in combat. After flying to the 25,000ft altitude of the US bombers, it had only a few minutes to track them down before running out of fuel. Even if it did manage to find the bombers, it had the equally prickly problem of lacking a weapon which could be aimed effectively while closing on the target at some 400 mph difference in speeds.
Dr. Wurster of Messerschmitt sugegsted the answer in the form of Flak Rakete 1 in 1943. Instead of tracking down the planes, the rocket would be flown just in front of the target and then explode a gigantic 500kg warhead. This neatly solved the weaponry problem as well. Better yet, with no need to limit takeoff accelerations, the rocket could use solid fuel boosters for added thrust, further reducing the amount of fuel needed to climb. The result, even with this heavy warhead, was that a much smaller airframe was needed to carry the required fuel. So small in fact that the design could be portable and launched from a modified 88mm Flak gun mounting.
The design made as much use of wood as possible, due to the need to conserve other "strategic" materials in the rapidly deteriorating war situation. For the same reasons a new type of Walter rocket was envisioned, a modification of the engine in the Me 163 that would burn coal-gas (benzine) instead of the hydrogen peroxide used in other Walter designs (although a small amount of peroxide was used to drive the fuel pumps).
The missile would be guided primarily under radio control from the ground. The operator would fly the missile into the vicinity of the bombers, then cut the engine and glide it the rest of the way at a slower speed. This presented a real problem in the Enzian. Other German missiles were high-speed designs that could be flown directly at their target along the line of sight, easy enough to do even from the ground. The Enzian would instead be approaching its target from somewhere in front, which is considerably harder for the operator. Many experiments with radio and wire-guided missiles had demonstrated real problems with last-minute terminal guidance corrections in these cases.
The initial plans for solving this problem were rather advanced. The large airframe left plenty of room in the nose, which they intended to fill with a self-contained radar unit called Elsass. In the short term it was planned to use some sort of proximity fuse while flying the missile though the bomber stream. The warhead, of which several were studied, was to have a nominal lethal radius of 45 metres.
Several studies on the basic design were carried out, resulting in the FR-1 through FR-5. The FR-5 was considered to be a reasonable starting point, so development commenced on the newly-named Enzian E.1 (and its engine) in September 1943. By May 1944 sixty airframes were complete, awaiting their engines. In order to gain flight test data they were fit with RATO units instead.
A series of 38 flight tests commensed with generally favourable results, but the engine still lagged. Finally Dr. Konrad, designing the engine for the Rheintochter missile, was asked to modify his design for the Enzian. After study it appeared this was a much better (and cheaper) solution anyway, and after January 1945 there were no plans to use the Walter design any more. The resulting E.4 version with the Konrad engine was considered the production version.
During the flight test period it also appeared that none of the exising proximity fuses would be useful in the Enzian. A new design known as Madrid then gained favour, and it is for this reason the Enzian remains famous. The Madrid used an infra-red photocell mounted on the front of a steerable telescope mirror, with small metal vanes placed in front of the cell in the shape of a cross. By moving the mirror side to side (or up and down), the vanes would block off more or less of the image of the target, and the system continually moved the mirror in a both directions while attempting to find the direction that maximized the signal. This kept the mirror pointed at the target. The missile's control system then had the task of attempting to point the missile in the same direction as the mirror. However the system was never actually developed beyond a test-bench mockup.
On January 17th 1945, all ongoing projects were cancelled by the Luftwaffe in order to concentrate all possible effort on only two designs, the Messerschmitt Me 262 and the Heinkel He 162. This wasn't the entire story, as many in the Nazi and Luftwaffe hierarchy had their own pet projects continued. However the Enzian was judged further from completion than Henschel's Schmetterling missile at the time, so it was cancelled. Messerschmitt engineers continued some low-level work on the project, hoping it would be re-funded, but by March it was clear the order would not be recinded (although it had been for other designs) and all effort ended.