One of the problems for the German military, and indeed any mobile military force, is the weight of the artillery and, more importantly, its ammunition. In traditional combat two forces would meet on the battlefield and then wait while the artillery was brought forward to settle the battle, notably if one side was in prepared defenses.
Of course this hurry-up-and-wait was exactly what the Blitzkrieg was attempting to avoid, by moving so quickly the enemy forces would not have any time to organize a defense. However this meant that the infantry would be facing forces that were dug in and provided with artillery support, with no such support of their own. In the opening stages of World War II the Luftwaffe was so overwhelming that they were able to address this by providing "flying artillery" in the form of the Junkers Ju 87 Stuka dive bomber, but this was an expensive solution to the problem.
A better solution would be very long-range artillery, organized into the army or corps level instead of the battalion. Units facing dug-in troops would call in artillery from far to the rear, and the artillery would only have to be moved after the troops had moved fairly long distances. This would also mean that a single supply line and organizational group would be able to provide fire support to the entire army, greatly reducing the logistics required. However this is difficult to achieve, as normal artillery grows in weight dramatically as the range is increased. Artillery capable of supporting an army over a front of, say, 150 km, would be considerably heavier and slower moving than a number of smaller guns. The longest range systems until that point had been the World War I Paris Guns, which had a range of just over 100 km but were so huge as to be completely immobile.
The solution was the rocket. Rockets can be made to fire to any appreciable range, but their weight scales roughly linearly instead of exponentially with range (at least for shorter range systems). On the downside, rocket artillery was notoriously inaccurate, a problem accentuated with increased range. Although a rocket might have the range to replace a large gun, it was not clear that it would be able to hit targets at that range. Rheinbote was built in order to test that question.
Developed in 1943 by the Rheinmetall-Borsig company, the first test flights were carried out that year. Several changes were made to the system, but the basic design remained the same: a long and skinny rocket stabilized with fins at the extreme rear. The Rheinbote carried a 40 kg warhead to an effective range of 160 km. The final version consisted of a four-stage rocket fueled by diglycol propellant, and reached over 220 km in testing. For shorter ranges some of the stages could be removed. It was launched from a simple rail on a mobile trailer. Over 220 were constructed and fired against Antwerp between November 1944 and the end of the war.
The concept of long-range artillery rockets on the battlefield would remain undeveloped after the war. Even Rheinbote was not used in its intended role, but instead as a smaller version of the V-2 missile in the strategic role (for which its 40 kg warhead was essentially useless) due to its poor accuracy. It was not until the introduction of lower-cost guidance systems that rocket artillery was able to provide the accuracy needed at longer ranges, and since the 1970s such systems have become a part of many armed forces