In his book, The Blitzkrieg Legend (original title Blitzkrieg-Legende, der Westfeldzug 1940), Karl Heinz Frieser wrote that during the campaign in France, blitzkrieg was just improvised, and that even the HQ was concerned about Rommel's position, since most of the generals had a classic conception of war operation, due to their experience in WWI. So was the blitzkrieg a true doctrine or was it just a myth to establish the legend of the invincible Wehrmacht?
So was the blitzkrieg a true doctrine or was it just a myth [...]?
I would not call it a myth. Blitzkrieg was the name people gave to what really happened. However, you will not find a single German document calling their doctrine Blitzkrieg.
If you watch History channel where they try to press 20 years of history in 30 minutes air time, you will learn that "Hitler invented Blitzkrieg, built superior tanks and declared World War 2". This is so simplified that you can only call it wrong. That indeed is a myth.
Hitler never declared nor intended WW2. His goal were a series of local conflicts, being won by Germany one after another.
German tanks in a one-on-one comparison were never superior and often inferior to their counterparts while Germany won it's "blitzkrieg" victories in France and early in Barbarossa.
So why did Germany win those campaigns? Was the so-called Blitzkrieg a German doctrine?
It's hard to get to a definite yes or no on that. Tank warfare was relatively new on the battlefield and all across Europe different people in different countries developed doctrines on how to use this new weapon. There were two main lines of thought: the rather conservative line of thinking wanted to put tanks as an infantry support weapon. Basically improved trench warfare with bunkers-to-go (aka tanks). Another, more progressive line of thought was to focus their power and achieve and exploit local victories.
Every country had their advocates for each line of thought. Charles de Gaulle for example was a french advocate for maneuver warfare. But the majority of the french general staff clung to their old tactics. Heinz Guderian was a German advocate. In Germany, this was more successful. While the general staff was distrusting, it was still either trusting enough or desperate enough to give it a try.
So Germany put a lot of effort into having their tanks focused on a Schwerpunkt and deploying them en masse and having their combined arms support motorized or later mechanized as well. Communication and cooperation between the tank forces and the other parts of the army and air force was essential and part of the doctrine. Good practice in maneuvering in large formations, better tactics and better equipment (not weapons or armor, but for example radios to communicate in every tank) led to victories over enemies that tank-by-tank on paper should have been superior.
You will find this as official doctrines and in books like Achtung Panzer! published well before the war.
However, nobody called this "Blitzkrieg".
People saw the results of the doctrines in action and gave them this name. You will not find this name in German documents or communication.
It also was not a well oiled machine. As any plan in war, it came into action and quickly fell apart only to be replaced by improvised battlefield solutions. But that happens to any plan in war. There is no army that works as a well oiled machine, no matter which soldier you ask.
However, the German army employs a doctrine called Auftragstaktik. That means compared to regular command structures, where commanding officers give their subordinate officers very detailed orders to execute, in the German military it was (starting way before WWI and it still is) normal to give the subordinate officers and NCOs a mission a timeframe and resources and let them decide on how to achieve the mission in the timeframe and with the resources given. This may look a lot like improvisation to people used to a more order-oriented command structure. This kind of improvisation, adapting the bigger pictures to the needs of the people on the ground, has been a German doctrine. So yes, much of the German victories has been improvisation, but it had been planned that way.
Yes, the German army employed tactics on a large scale that caught the allies by surprise. That was well planned, executed and ended in successful improvisation based on unfolding events like any successful plan. You could call that "Blitzkrieg" and it would not be wrong.
What you see on history channel, that someone with a master plan of "Blitzkrieg" overran a half dozen countries with a perfectly functioning army executing a plan flawlessly? That is a myth.
Frieser is certainly correct regarding the term and its original meaning. "Blitzkrieg" appeared frequently in newspapers, magazines, and a few books prior to 1 September 1939 and was used to describe the concept of a knockout blow, a war of short duration. As early as 1937, the word appeared in the Viennese newspaper Gerechtigkeit ("Die 'Schwarze Front' sieht schwarz," 21 January, p. 5. View at ANNO Austrian Newspapers Online, Austrian National Library). Frieser mentions its use in a 1935 Deutsche Wehr article as well. Of particular note, Lt. Col. Viktor Braun used the word to describe a short war in the German army publication Militaer-Wochenblatt in December 1938: "Nach den Zeitungsnachrichten hatten die diesjaehrigen franzoesischen Manoever den Zweck, die Bedeutung des strategischen Ueberfalls--auch Blitzkrieg genannt--zu pruefen." General Georg Thomas, head of the War Economy and Armaments Office of OKW, used the word in May 1939 in a talk to members of the German Foreign Service. He described it as a war lasting "days and weeks." A copy is among the Nuremberg documents. I addressed all of this in my 1997 article in The Journal of Military History ("The Origin of the Term 'Blitzkrieg': Another View.") I found approximately forty instances prior to the war in which the word was used and, with one exception (August 1939), they had nothing to do with tactics and doctrine. The association of the word "Blitzkrieg" with such was largely a creation of journalists and postwar historians.
Take a look on the 7th Panzer Division (Wehrmacht) which earned the nickname ghost squad (Gespensterdivision). They rushed 150miles in 24 hours.
I think, actions like these are responsible for "Blitzkrieg" myth.
It was a tactic, but not a doctrine.
I read a lot to find out which doctrine the Germans are have on ww2 and there was no offical doctrine, in my opinion. "Mission-type tactics" (Auftragstaktik) or "leading by mission" (Führen mit Auftrag) is nearest to a doctrine.
Found a very interesting source here, its a PDF which is called "German Tactical Doctrine".