So did Douglas company, which was striving for a landplane with the range to cross oceans. That flying boats were not really suitable and would be aced out by sufficiently capable land planes pretty soon was a common position back in the late '30s.
To be sure, the wartime investment in huge and hard-surfaced runways was most helpful to the postwar land plane market. As an airship fan, I like to introduce this other variable and argue that if combined airplane/airship operations had been developed then one possible solution to the problem of super-capable land planes needing super runways might be for the airships to become the nexus of exchange between surface and long-range air routes, via STOL airplanes from numerous grass fields flying up, transferring their passengers and cargo, and ferrying down return passengers and cargo to local destinations, while long-range high performance airplane also hook on and passengers and cargo headed for distant destinations join these "trunk" operating lines.
If I must sweep aside this dream, which in some moods I freely admit is far-fetched--then I think that a lack of WWII taxpayer funded global airfields will just somewhat delay and slow the land planes a bit, and localities will simply have to pony up to make the runways. Since OTL there was also a massive glut on the market of DC-3 type planes and competitors, not to mention transports converted from bombers, the situation is entirely different. On one had, far far less developed infrastructure and general acceptance--on the other, the leading manufacturers and promoters are not facing a glut of airframes and choices of runways. They will then carefully target a few locations for being the best prospective airports, and develop them, offering a fair fit of numbers of new made airplanes to go with them. The land planes will shoulder aside the sea planes except perhaps in markets where the sea planes are the best fit.
Not that I am totally immune to the romance of the flying boat of course! Early jet planes suffered from having relatively low thrust at sea level compared to propeller planes (with any power plant) and thus, as planes in general required in the era before flaps were developed, required long relatively slow takeoff runs. Thus just as WWII hardened, relatively long runways built for war service OTL supplanted grass fields, these were in turn dwarfed by the requirement of lengthening them a lot more to allow jets to take off and land. Early jets did tricks like water injection into the compressor which (by evaporative cooling) allowed higher densities to be reached, in turn allowing more fuel to be injected and thus raising the thrust at takeoff (a side effect being tremendous smoke production, and also more noise) or as on the early De Havilland Comets, used auxiliary rocket thrust--the Comets had hydrogen peroxide monopropellant rockets, which I think could easily be upgraded to burn some kerosene, such as jet fuel--even burning fuel rich "ker-oxide" rockets use pretty low ratios of fuel to oxidant.
So I envision a trans-oceanic floating plane, more of a flying raft than flying boat, with a largely flat-bottomed delta form and hydrofoils that can be retracted or extended, submerged. Such a jet would start out simply floating as a raft, then with thrust the foils would push the hull clear of the water at a relatively low speed so it does not have to be aggressively optimized for water streamlining, and accelerate mostly clear of the water to takeoff speed. It might be necessary to have 2 stages of foil, a set of big ones for initial speed and then retract those, leaving much smaller ones that have minimum drag at a much higher speed. And perhaps also to have auxiliary thrust of some kind, such as from takeoff rockets. The idea is that it can take advantage of very long takeoff runs on open water. Technically it would be amphibious as adding a set of wheeled landing gear would not be too burdensome, but mainly this would be a safety measure allowing the plane to put down on an emergency basis at a land runway--probably it would also be necessary to pack an emergency braking parachute set. By and large the designers would not expect to find runways long enough for it to operate and it would mainly act as a seaplane.
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What kind of world could avoid WWII? I actually think this is pretty easy, in the context of Europe if not east Asia anyway--just butterfly away Hitler. I know that the Nazis drew on very broad circles OTL, but I do think that without a rare figure like Adolf Hitler, it might be very difficult for the plethora of right-wing movements to be united under single leadership even with a conservative like Hindenburg trying to midwife this in a crisis like the Depression. The aristocrats would look down on plutocrats and masses alike; rich capitalists would have trouble getting anyone to follow them; the masses are split between far left and far right. I think if Hitler had died on the front during the war, it is entirely possible no one could manage the coalition he did and German politics would be somewhat deadlocked in the Depression, favoring a conservative regime but not permitting the sort of sweeping authoritarianism that allowed the Nazis to ban all opposition OTL--the Social Democrats would survive the degree of repression that would be politically feasible, and the Communists would at least persist underground and probably be allowed (grudgingly) to return to open electoral politics someday.
Perhaps some form of national monarchy would be restored. What might be the prospects of some house other than Hohenzollern being offered the imperial crown, under someone like Hindenburg? A conservative block controlling Germany would have to include the Catholic Centrum Party--might this open the way for a Catholic dynasty? If not perhaps the Hapsburgs, maybe Bavaria's Wittlesbachs? This would pave the way toward an Austrian Anschluss too since the southern Germans might be seen as a conservative bloc to outweigh Germany's own Socialists, causing the Lutheran northern aristocrats to swallow their objections to a Papist dynasty--the monarch's power would be circumscribed after all, insofar as it is dictatorial depending on the coalition of all German rightists.
Without Hitler and a defiant rearming Third Reich, I think western Europe could easily remain at peace; there might be trouble in the east and Balkans especially, but it could be managed and limited. In particular I think the vast potential threat of the Soviet Union would never be mobilized for mass war (Stalin being IMHO the Great Procrastinator, due to his political situation in Russia) but it would remain a terrible scarecrow, and this would probably motivate the conservative powers to be more cooperative.
Overseas it is another story of course, with Japan being almost certain to embark on a similar program to OTL in China, which puts her at odds with the USA. This threat more than the Soviet might enable considerable US rearmament even if the war with Japan never goes hot--far below WWII levels of mobilization even if the war does break out, but "A Navy Second to None" will be joined by a fairly strong if unexperienced Air Force, per popular doctrines of the era. All this gives a bit of a Keynesian pump priming. I believe the Depression was inevitable, pretty much, due to deep structural factors, but also the recovery from it would be coming along eventually. Part of what happened to make the 1950s a buoyant era for the West was technical and structural, having to do with substitution of high-tech chemical products for traditional Third World imported plantation products that had made the colonial era so necessary and profitable.
I know people say the European colonies were not economically profitable, but what I believe happened there was that the parties making the biggest profits from it largely evaded paying their share of the costs sunk into the colonial system. On paper, looking at the accounts of the colonial governments, they were in the red, spending more than they appeared to take in from the subject peoples, but in reality the labor and resources of these lands were being profitably exploited by private industry in the homelands, and thus the central governments were more than willing to go on bleeding tax revenues--paid to a great extent by the working classes after all, but these too recognized that their jobs and paychecks depended on the colonial system.
In the new evolution of global capitalism that would emerge even without the war after the 1930s, though, it was increasingly possible to substitute industrial products produced in the First World nations for increasingly many colonial inputs, and this allowed the market to undercut the prices paid for the latter, further intensifying the exploitation of Third World mostly colonized peoples under the table as it were. Another response was to invest increasingly in industrializing the Third World, which would later mean heavy competition for the First World domestic economies but at the time seemed like a political and economic win-win. Globalization of industrial production was a by-product of developing infrastructure for the import of First World goods anyway--if the rich nations invested in the ports and roads for bringing in First World manufactured cars and other machine goods and mass quantities of other consumer goods, the roads and ports existed to ship products of those territories back to the global market!
I would think that anti-colonial conflict would be on the rise and continue to intensify until legal independence of the colonies was on the agenda. India, despite its vital central position in the British system, had already reached that point by the eve of WWII OTL. The rest of the Asian colonies would soon follow and then Africa, I think. Japan of course complicated the situation in eastern Asia considerably. But without a European war to paralyze the colonial powers, I don't think the Japanese would have been so rash as to dare their OTL escalations of 1940-41. Perhaps though in the context of simmering unrest in the colonies, they might venture to horn in, posing as the champion of the Asian? It would b dangerous for them and I suppose maybe, if they felt the Western powers were too much on the ropes to respond effectively, they might move as OTL, but somewhat later? Or possibly the Sino-Japanese crisis would move to some resolution, probably a terrible one for the Chinese, perhaps a de facto partition between Japanese controlled eastern Chinese territories and a strong but peripheral Communist regime in the west?
OTL the tremendous injection of investment, in war-rationing Britain and the arming USA, caused by the war most certainly did jumpstart the new wave of industrialization that characterized the 1950s and '60s. That said, I do think that there are enough political catalysts in a global situation where Japan and the USSR appear to be the greatest threats to settled peace, just enough to justify some fundamental R&D and for companies taking advantage of new tech to pull ahead of those that don't, making private R&D perhaps more popular than OTL where governments could be counted on to fund it--or perhaps creating the political basis of state-funded research despite such an overwhelming threat as the Third Reich being lacking. The Communist and Japanese threats might well be sufficient justification in Washington, or London.
So, I expect an initially sluggish recovery in the early 1940s to pick up momentum and mushroom into a strong boom toward the end of the decade. There will continue to be short term crashes and reversals of course, probably continuing every decade like clockwork, but the crashes around 1950 and 1960 would be mild (they'd scare hell out of everyone who remembers 1930, but it would never get anywhere near that bad) while the booms in between would be long and buoyant.
In these circumstances I believe aviation would forge ahead with relatively little delay versus OTL. After all it did in the 1930s despite the misery of the Depression; compare the planes one could find in any developed world major airport in 1930 versus those of 1938! OTL air forces were so transformed that top of the line aircraft of 1930 were obsolete junk in '38, and the best planes of that year were just about to be eclipsed in turn. Here with the world at peace for the most part I would think the military planes would be changed much more slowly, but the civil sector was demanding and getting rapid advances every year, and that would continue.
I suppose that for some time, perhaps to as late as 1950 or so, there would be some deficiency relative to OTL in the number of aircraft in operation. Instead of tens of thousands being made frantically (and shot down almost as fast) there would be what a slowly at first expanding market would bear to purchase for profitable use. OTL would boast far more people who had flown in an airplane than the ATL would in 1945; surely one factor among many fostering explosive growth of the airline industries and other private plane use was soldiers by the legion growing accustomed to flying, if not indeed jumping out of them with parachutes on. This ATL would lack that; only those rich enough to freely choose buying a plane ticket as an option among many other choices would know the experience.
But, I don't think the ATL would lag tremendously in quality of aircraft. If anything, the lack of a feverish wartime demand to produce as many standard issue models as possible would mean the favored manufacturers and designers of OTL would not have the tremendous fortunes they got here for being chosen--but all the more room then for their competitors, with their own innovative notions. Probably OTL did favor the best of the best, or anyway planes whose virtues put them in the top ranks. But there is no telling what odd improvements might have been stampeded in the rush of war. The aircraft industry would not be accustomed to making huge quantities of a model at a bargain price; airplanes would be more expensive--therefore the various dozens of competitors edged out OTL by the handful of super-manufactures would be getting better payment for their alternative designs thus sustaining more diverse approaches somewhat better.
Meanwhile though the mass market will be rising. Planes like the DC-3 that excel because they hit the sweet spot of cost-effectiveness would be edging their way forward and their designers would be learning to make, if not thousands, than anyway hundreds, of their best. The prices will be coming down, the reliability and safety of civil aviation improving, the numbers of passengers rising every year and the economic status of the poorest passengers falling toward average levels or below; few ordinary people will have ever flown but more and more of them could claim that every year.
By 1950 then, the number of flights and number of planes in long distance airline service would probably pull up to roughly the same levels as OTL of that year. The types of planes in civil service would be more diverse, but essentially all of them would be purchased for their exact role, not repurposed war surplus transports.
What about jet propulsion? Surely that will languish, due to the obvious facts that in OTL jet engines were rushed into military service long before they were really suitable and reliable, and due to their quirks and characteristics would take a lot of work to make suitable for long distance passenger service? Did not Boeing benefit enormously, in developing the 707, from a couple decades of military contracts paying for development of two jet bombers, building on their experience with the B-17 and B-29 to boot? Did this not enable Boeing engineers to become expert on many recondite and cutting-edge aspects of high altitude, high subsonic speed transport design at the extravagant expense of the US taxpayer, and had not those same taxpayers subsidized, through 20 years of military orders, the jet engine industry that only after those two decades had past could supply Boeing or Douglas--or DeHavilland or Vickers--with engines passengers could rely on?
All this is true. On the other hand, when Frank Whittle patented his turbojet design, he may have been thinking of military applications, being an RAF officer--but he surely was also thinking of civil applications. It had been well understood at least since the aftermath of the last glut of war-surplus airplanes in the early '20s that a major key to economical hence profitable air lines would be higher speed, and that higher speed would imply higher altitudes--but that the airplane propeller had already begun to hit the speed of sound limit during the Great War! Much could be done with improved aerodynamics, with variable-pitch props, with improved aero engines--but the sound barrier loomed, not so much holding back airplane flight speeds directly, but limiting the effectiveness of propellers. The idea that aircraft propulsion needed to be liberated from the limits imposed by props was widespread, and Whittle, thinking on the problem, had developed a solution. So had others--after the war of OTL, when Whittle met his German counterpart Ohain, he quickly accepted as fact that the German was unaware of his patent and had invented his version of turbojet on his own. Meanwhile also, Soviet investigators too were closing in on the turbojet solution, and in the USA designers at Lockheed were sketching prospective jet airplane designs, being quite confident the industry would soon provide the missing power plant. The fact is, if Whittle and Ohain would both have gone begging due to the shortsightedness of the Air Ministry in Britain and the lack of war preparations capital and priority in Germany, sooner or later some American would come forth claiming to invent the turbojet, and will have done so on the solicitation of some firm intending to sell airplanes to airlines. It might indeed be a good decade or two before general knowledge of the high-subsonic flight regime and general ability to make an engine that could reliably work for days and months of continual service would be forthcoming. But I think in the interim, interim designs with some drawbacks but still competitive in the general aviation market would go ahead and incorporate jet engines.
After all, there was a clear line of evolution, visible in retrospect, from the turbosuperchargers developed by conventional engine makers in the 1930s to provide more power at higher altitude, to the turbo compressor.
In view of the fact that it took over two decades from the inception of the first prototype turbojets to adoption in profitable airline service, should we expect jets to be delayed by some extra decades since the militaries of the various powers will not have the largesse to spend on fostering them through infancy, childhood and adolescence? Again I say--lower quantity, similar quality. The invention itself should not be delayed at all. The adoption for military purposes might be, but in the interim it will be the civil market designers taking care of it in infancy, and sooner or later the penny will drop for some air force or other. The number of orders these peacetime air forces can offer the manufacturers will be orders of magnitude smaller, and the engines will thus be more expensive, but the engine designers will still appreciate the subsidy of military development contracts, which will have to pay the bills well enough. There would be far fewer jet fighters in the ATL 1960, and maybe no bombers, but the few that exist will be pretty comparable to the OTL state of the art of that year.
In the long run I think avoiding World War II, and its associated devastation, will leave the ATL world better off in every respect, with technology more advanced than ours and getting relatively more so every passing year. I think this turning point, in aviation, will happen sometime between 1950 and 1960, so that in 1951 indeed world aviation would be backward, by some years, compared to OTL, but in 1961 it would be the ATL that is a bit superior, and by 1970, clearly ahead.
After that the ATL may run into a stagflation analog and perhaps it too will stumble and shuffle its way along in the last few decades of the century. But I think in any given year it will remain ahead of us.