I decided to come back to this after reading pbehn's informative comments on Operation Steinbock above. What this shows is that the Luftwaffe was fighting its own war without giving any thought to the overall situation. Very little thought would have told them that a successful Allied invasion of France would lead to Germany's defeat but the Luftwaffe did not make any serious plan to oppose such an invasion.
Of course the force of two engined bombers (and crews) that was expended in Operation Steinbock could not survived if they had tried to attack the invasion fleet by day. However, attacks at night might have at least caused some confusion had the crews undergone some training and had the bombers been equipped with radar (June 6th 1944 was a full moon but the weather was close to stormy) and had some technique been developed for using flares.
"But how could they know that an invasion was imminent or that it was aimed at Normandy?", I hear someone ask. Well back to the Me 262. Only a dozen Me 262s flying over the South Coast of Britain every day would be enough to roll back the curtain concealing Allied plans.
So we return to asking when the 262 could have been put into production. My guess is that production could have started very early in 1943, squadrons could have started training in Autumn 1943 and several hundred aircraft might have been available by June 1944. The Me 262 was aerodynamically ready by early 1943 and the aircraft tested by Galland in May 1943 was very similar to an early production 262 apart from the engines.
Which brings us to the engines. According to
Luftwaffe Resource Center - German Luftwaffe (Air Force) from 1935-1945 - A Warbirds Resource Group Site "By Summer 1943, the Jumo 004A engine had passed several 100-hour tests, with a time between overhauls of 50 hours being achieved." The article goes on to say "However, the Jumo 004A engine proved unsuitable for full-scale production because of its considerable weight and its high utilization of strategic material (Ni, Co, Mo), which were in short supply." Now it is certainly true that cobalt was unavailable after Operation Torch. However, chromium was being imported from Turkey and nickel from Finland up to Summer 1944. Germany's molybdenum was produced at the Knaben mine in Norway and production was disrupted by air attacks in March and November 1943 but the Jumo 004A contained little molybdenum which was a critical component of armour for tanks. The Jumo 004A may have been heavier than the 004B but it was more powerful as it spun faster and its weight did not greatly reduce the performance of prototypes. It does seem likely to me that a Jumo 004A after a slight redesign to remove cobalt could have been produced from June 1943 to June 1944 in fairly large numbers without straining Germany's resources.
There is some very weak support for this in "Engines of Desperation: Jet Engines, Production and New Weapons in the Third Reich" by Hermione Giffard, Journal of Contemporary History 48(4) 821-844, 2013 which has "In the case of the jet engine, lower quality (made worse by shoddy construction work) was accepted, even recommended - as a way to ease production regardless of the known and dangerous consequences of doing so." Unfortunately, I don't know if the author meant the 004A to 004B decision or the transition from piston engines to jets and I haven't yet worked out how to get her thesis. She references Schabel's Die Illusion der Wunderwaffen but my German is poor and the parts at
Die Illusion der Wunderwaffen seem to mostly say the same as Walter J. Boyne's article "Goering's Big Bungle" at
Goering's Big Bungle | Air & Space Forces Magazine.
If we want to know how quickly an efficient organization can bring an aircraft into production, we can look at the Wikipedia article on a nearly exact contemporary of the Me 262:
The Cyclone-powered XF6F-1 (02981) first flew on 26 June 1942, followed by the first Double Wasp-equipped aircraft, the XF6F-3 (02982), which first flew on 30 July 1942. The first production F6F-3, powered by an R-2800-10, flew on 3 October 1942, with the type reaching operational readiness with VF-9 on USS Essex in February 1943.
First off - beware of anything that note Wikipedia as one of its sources. Wikipedia - particularly on the subject of the Second World War and German technology is incredibly unreliable - infested with everything from those believing the old wives tails to full-on Robert Arndt level S-Tier Wehraboos.
Everybody who was thinking rationally knew that a successful invasion of France would lead to Germany's defeat. Everybody who has a clue about Industrial Economy and Logistics knew that a serious Allied invasion was going to succeed. But - Germany had never, in its existence had a number of people with a good grasp of either.
The Luftwaffe certainly had a plan to deal with the upcoming Allied invasion of France. They could not handle the intense Counter-Air offensive over Western France, that made their what would be forward airfields untenable - so they stocked up the airfields, and pulled their fighter and strike units back beyond allied fighter-bomber and medium bomber range, ready to stage them forward when the Invasion came. It didn't work. Their scheme to stage the aircraft forward consisted of immediately deploying their fighters, with their Crew Chiefs stuffed in the baggage compartments, to the forward airfields - no transport aircraft, and ground transport was too slow - and effectively interdicted by the Allies - Fighter-Bombers may not have been very effective against tanks, but trucks, trains, even bicycles, could not move in daylight without getting swarmed. The result was when the German Jabos showed up, they were massacred - out of fuel, unarmed, the Crew Chiefs in the hold unable to escape, and the Pilots unwilling to abandon them. Any that did deploy were swarmed as soon as they moved.
It took weeks for those traveling by ground to get there in any organized fashion.
Now - Jumo 004 had more problems than just weight and requiring strategic materials - while there were sources of some of them, transportation was beyond difficult - the Baltic, the Mediterranean, and the Black Seas were effectively blockaded, and had been since early 1943. Land routes are long, circuitous, and, in the Eastern sections, run fairly close to the combat zones - where they not only are at risk of interdiction, but are competing with supplying the fighting.
The Jumo's compressor aerocynamics were, to put it mildly, crap. Abysmal Pressure Rise per stage, low efficency, and unstable aerodynamics that induced Compressor Stalls when exposedd to anything but ideal conditions. The hot sections weren't any better - The burners were poor (If you've got flame coming our of the back of a non-afterburning jet engine, you're Doing It Wrong - it means you're not properly burning the fuel you're pumping in) - They came up with a very clever solution to having to use lower temperature materiels - air cooled turbine blades, with cooling air ducted up through the blades - something everybody uses now. Then you fold in the German's poor record as regards Mass Production. Even when they were pushing large numbers of unusable airframes out the factory doors, quality was an issue - with their methods requiring lots of hand-fitting, very little real parts interchangability, and, due to their policies of filling out the factories with forced labor and slave labor, sabotage on a large scale.
Comparing U.S. and German experience in developing and introducing new aircraft is comparing Apple to Cumquats. First - the U.S. has a much larger industrial base, was not reliant on conscript labor, and the Hellcat was an evolutionary desigh that used proven engines and other components. The U.S. had a huge and comprehensive training system that produced pilots able to fly the airplane, fly on instruments, (This is important, more later), navigate long distances, and had combat training from veterans rotated back to the interior to pass on their experience. The Germans were trying to feed in an experimental type, with radically new engines, and an incomplete flight testing program, into a system where, even if airframes were available, pilot training was inadequate - Day Fighter Pilots received, at best, rudimentary instrument flying training, and little navigation training. Training itself was, by late 1943, at risk of being jumped by Allied fighters. The Instructors, at least those who hadn't been lost flying the Stalingrad and Tunisia aerial supply efforts, were few and far between. Veterans were retained in combat until they were casualties, and air combat training was provided by the operational units.
When Kommando Novotny finally began service tests in late summer 1944, with experienced pilots, they lost more aircraft and pilots to operational losses than combat. (And combat losses were high, as well)
As for the "dozen Me 262s flying recon over England" - where are you going to base them? The 262's not long-legged. They'd have to be based on the French Coast. Which, like pretty much all of Western Europe is under constant Photo-Recon cover. We already knew about the Me 262 at that time - and a jet base can't be concealed in that era - it required a fairly long paved runway, and the jet exhausts left unique and easily identifiable trails on the pavement. What would have happened, as happened in late 1944-45, is that those airfields would have been bombed by Allied Light and Medium Bombers on a regular basis, and Allied Fighters would be swarming around them, waiting for a jet to take off or land - where the Piston Fighters have a significant performance advantage over the jets. (Below about 250-275 mph, the Prop FIghters have a significant Excess Power advantage, which turns into higher acceleration, climb, and sustained turn rates - the slow spoolup times of the jets aren't relevant). The other most likely consequence is that the Meteor goes into squadron service earlier, and the Vampire gets higher priority.