I'm uncomfortable with saying "necessary", as playing with counterfactuals like that is risky, but we can certainly say it was of vital importance. If we compare the comparative economic and industrial strengths of the US and the Soviets, especially with a look at where their focus was, and what areas Lend-Lease was assisting with most, it certainly does demonstrate that Land-Lease was not something to write off as inconsequential. I've written a bit about this before so am cribbing off of that.

The US certainly had the larger overall capacity, but that doesn't mean they outperformed the USSR in all categories. But neither does USSR outperformance necessarily point to their dominance!

Raw Materials/Food Percentage World Production in 1937 (Ellis)

Production	US	USSR	Germany^1	World Total (million metric tons)
Coal	34.2	9.3	15.3	1,247.4
Oil	60.4	10.6	0.2	272.0
Iron Ore	38.0	4.0	4.1	98.0
Copper Ore	32.4	3.3	1.3	2.3
Manganese Ore	0.7	40.5	8.4	3.0
Chrome Ore	0.2	15.3	-	0.6
Magnesite	10.6	27.2	27.9	1.8
Wheat	15.2	26.5	4.7	167.0
Maize	55.2	2.4	0.6	117.4
Beets	15.7	22.7	24.7	9.7

1: Includes Austria and Czechoslovakia

That isn't all of the categories, in fact I left out 13 raw material categories, and 3 food, all of which the United States was superior to the USSR in (Lead, Tin, Rice, Meat, etc.). What I'm showing here is the that the US was clearly far superior to the USSR in most of the major categories for raw materials, with the USSR having higher production in only a small number of things - all of the ones they were higher are shown here - and not ones that are most vital, like coal.

Also keep in mind that these numbers are from 1937, so represent pre-war production, so the US would be unaffected, while the USSR would suffer setbacks in losing a large chunk of territory. For instance, in 1941, producing 151.4 million metric tons of coal, the USSR would drop to only 75.5 in 1942, and still didn't hit pre-war numbers by 1945 (149.3), while the US remained steady around 525 mmt through the war.

As for overall industrial capacity, again the US is just far and away beyond the USSR.

1937 National Income and Percent on Defense (Kennedy)

Power	National Income in billions of dollars	Percent spent on defense
USA	68	1.5
USSR	19	26.4
Germany	17	23.5

First, here is a look at pre-war income and defense spending. The USSR had higher defense spending, being in the midst of modernizing a large standing army (while the US maintained a very small military force), but in doing so was spending 1/4 of their total income in the late '30s! In terms of world manufacturing, while the USSR had improved markedly over the decade before the war, they still trailed far behind the US.

Percent shares of World Manufacturing Output, 1929-1939 (Kennedy)

xxx	1929	1932	1937	1939
USA	43.3	31.8	35.1	28.7
USSR	5.0	11.5	14.1	17.6
Germany	11.1	10.6	11.4	13.2

So the USSR was certainly improving their manufacturing capacity relative to the US but they were still a far ways off, and as Kennedy notes:

The key fact about the American economy in the late 1930s was that it was greatly underutilized.

As he goes on to point out by way of example, while the US was producing 26.4 million tons of steel in 1938, itself a notable amount above the USSR's 16.5 million, by that point the USSR was working at maximum capacity, while the US was outproducing them with fully 2/3 of steel plants idle! Additionally, with unemployment running at ~10 million still in 1939, the US was able to both mobilize for war, inducting over 16 million men and women into uniform during WWII, and still push production into massive overdrive vis-a-vis peacetime production. Agricultural output, for instance, reached 280 percent of pre-war yield!

Overall Kennedy rates the 1938 relative "war potential" (a metric of comparative strength he admits is somewhat imprecise) of the seven leading powers thus:

**"War Potential" in 1938

Country	Percent "War Potential"
United States	41.7%
Germany	14.4%
USSR	14.0%
U.K.	10.2%
France	4.2%
Japan	3.5%
Italy	2.5%

The US dwarfs not only the USSR, but any given nation 3 times over.

So now let's look at what this meant once war broke out.

Total wartime production numbers in million metric tons (Ellis)

Item	US	USSR	Germany
Coal	2,149.7	590.8	2,420.3
Iron	396.9	71.3	240.7
Oil	833.2	110.6	33.4 (+23.4 synthetic)
Steel	334.5	57.7	159.9

I think you get the point. The US was a head above everyone else. In all those categories the US makes up at least half of total allied production, and alone surpasses or near equal total Axis production. But enough with raw production, I'm sure you want the weaponry!

Total wartime production numbers for select weapons systems (Ellis)

Item	US	USSR	Germany
Tank/SPG	88,410	105,251	46,857
Artillery	257,390	516,648	159,144
MGs	2,679,840	1,477,400	674,280
Trucks	2,382,311	197,100	345,914
Planes (all types)	324,750	157,261	189,307
Fighters	99,950	63,087	-
Bombers	97,810	21,116	-
Merchant Shipping	33,993,230 tons	???	???

Munitions production by year, in billions of 1944 dollars (Rockoff)

xxx	1940	1941	1942	1943	1944
USA	1.5	4.5	20.0	38.0	42.0
USSR	5.0	8.5	11.5	14.0	16.0
Germany	12.0	6.0	6.0	8.5	13.5

I left out naval production, aside from merchant, as the USSR had negligible production (70), while the US built over 1000 combat ships and subs. While the USSR, as you notice, does have higher production in tanks and tubes, this is a bit deceptive. The US actually out produced the USSR in tanks in 1942 (24,997 to 24,446) and 1943 (29,497 to 24,089), but while production was ramped down by the US to only about half of peak in 1944 (17,565), the USSR continued to increase production through that year but never topped the US peak production (28,963).

So while they made more tanks, it doesn't necessarily represent higher capability exactly, but priorities of production. In fact, although Germany's surrender in spring of 1945 sped up the process - Ford's B-24 plant at Willow Run, for instance, being slated for shutdown on August 1, 1945 - the process for slowing down production and increasing non-war manufacturing was being planned by late-1944, when the War Production Board agreed that auto manufacturers, who had suspended commercial production by early 1942 to focus on war needs such as tanks, trucks, and planes (and accounting for 20 percent of total US production during the war!), could begin to plan return to their normal production, which resumed before the war was even over, with Ford alone producing just shy of 40,000 cars in 1945, beginning in July.

As you can see with the second table that breaks down by year there, once the US ramped up production, it really was the waking giant of so many pithy quips. That the USSR out-produced in a small number of categories looks considerably less remarkable when considering how much more, and how much more diverse, American production was (For instance the Manhattan project, which, while estimates are not exact, cost somewhere around $1.89 billion dollars, but was less that one percent of total defense spending during the war).

Additionally, one of the most important factors to not overlook is trucks. To quote David Glantz from "When Titans Clashed":

Lend-Lease trucks were particularly important to the Red Army, which was notoriously deficient in such equipment. By the end of the war, two out of every three Red Army trucks were foreign-built, including 409,000 cargo trucks and 47,000 Willys Jeeps. [Note, Glantz's 2/3 stat is a higher ratio than Ellis indicates, but Ellis still points to 2:1 import/production, and regardless there may be other caveats in play]

As for the domestic ones, almost all of those were licensed copies of Ford trucks anyways!

The importance of those trucks can't be underestimated. First, they were they of vital importance for the logistics of the Red Army as well as its motorization and increasing mobility. Glantz again:

Without the trucks, each Soviet offensive during 1943-1945 would have come to a halt after a shallower penetration, allowing the Germans time to reconstruct their defenses and force the Red Army to conduct yet another deliberate assault.

And while the core benefit of all those extra wheels was movement of men and materiel, while Soviet propaganda photos always showed them mounted on domestic built trucks, most of the fearsome Katyusha rockets also were mounted on American built examples.