B-29s were capable of a service ceiling of over 32,000 feet, but usually few much lower than that after problems of flying at such altitudes (jet stream messing up navigation and accuracy problems, already bad with B-17s and B-24s over Germany flying at 20,000-25,000 ft were that much worse at 30,000+, and I'm sure other issues).
I do suggest reading this post in a thread talking about what the Luftwaffe could've/should've done post Battle of Britain:
Do-17 production stopped in 1940 (?) Good. You need as many He 111s as you can get until you get your He-177 - 4 flown, tooled up and in production. I disagree, get the ju88 instead. They were getting into production. Problem is that it needs engines. The BMW 801 was a minimum I wonder...
The Ta-152 was ultimately developed for a threat that didn't materialize. But so was the Welkin (never operationally used and had problems of its own, namely the thick wing), and the various HF Spitfires, including the VI and the VII. The switch to the V-1650-7 in the P-51B/C/D/K after starting out with the -3 was because most Luftwaffe fighters didn't perform well above 20-25,000 ft. The -3 Packard Merlin ended up being a bit "too" high altitude rated (though at most alts from what I've seen there was little difference between a P-51B or D fitted with either engine until you got up to 25,000 ft, though the -7 did benefit from higher boost numbers later in the war that I don't think the -3 was usually operated at. at least by the USAAF).
And the Germans did give up pretty quickly on Ju-86 overflights after one partially successful interception over England (a modified Spitfire IX caught and chased one, but the Ju-86 escaped with only minor damage) and one totally successful one over Egypt (shot down by a modified Spitfire V at 49,000 ft). Maybe that convinced them that the RAF and probably the USAAF had high altitude fighters capable of shooting down high alt bombers, recon planes and other fighters.
And that did coincide with the RAF and USAAF scaling back development of high altitude interceptors, based on the fact that the overwhelming majority of German aircraft encountered could be easily dealt with by existing planes as far as altitude performance, and the USAAF ultimately killed their interceptor programs (XP-72, XP-67) or focused on multi-role long range fighters that could be converted into interceptors (P-51H, XP-82/P-82B, P-47M/N) after the Germans aborted the Amerikabomber program. For that, the USAAF feared that such a plane was possibly workable for the Luftwaffe, but didn't know until post-mortem how many problems that the Germans were having that crippled the program almost from the beginning.