It's interesting how much development went into DOS clones while Microsoft kept DOS at 3.x throughout the 80s. (MS and IBM expected OS/2 to replace it but that never really materialized in time.)
Concurrent DOS started as multiuser CP/M-86 with a DOS personality. It would be followed by a complete rewrite for the 286 and 386 in 1985. CP/M and DOS personalities, written in C, multitasking, and with loadable device drivers, networking, most of the modern OS expectations, really. Unfortunately compatibility with real mode DOS programs directly manipulating the hardware was spotty and the 286 support was broken on early Intel 286 revisions. This would eventually become DR DOS. It was the only DOS-like OS that actually supported multitasking (some) real mode programs in protected mode on the 286. Quite a tricky feat.
Between spotty compatibility, Microsoft trying to keep Windows from running on DR DOS, a high price and tiny msrketshare it never got mass adoption. Did see some use in business. The PC would not get such features in the mainstream until Windows in the 90s.
I'd have to disagree. The only relationship between the Concurrent DOS which had CPM roots, and the Concurrent DOS-286 (CDOS-286) which became FlexOS was the name 'Concurrent DOS'. i.e. marketing / branding.
That (CDOS-286 / FlexOS) did not eventually become DR DOS, DR-DOS developed from the old CPM-86 based thing. Both were active at the same time. I was using both between 90-97, and for a good portion of that time, both were being updated (though FlexOS moved to Novell ownership).
Otherwise your description of FlexOS is reasonably accurate. The 386 version still had a 286 core, but with some modules replaced, it used the 286 loadable drivers. I was working for a company using it as a (soft) real-time multi tasking OS, with multiple processes running.
The version I used no longer had the DOS support for 286 (it did for 386 - but limited), from the stuff on bitsavers, it looks like that 286 based DOS support was gone after version 1.3.
BTW - The second page of that PDF mentions porting Concurrent DOS to the 68k on the Motorola VME/10. I believe that is actually a reference to porting FlexOS (CDOS-286) to the 68k, as mentioned here:
If one wishes to attempt porting it to another 68k platform, one can find that from this page, in the 'CP/M-68K' section, titled 'CDOS for the 68000', the files cdos1.zip, cdos2.zip, cdos3.zip
Concurrent DOS started as multiuser CP/M-86 with a DOS personality. It would be followed by a complete rewrite for the 286 and 386 in 1985. CP/M and DOS personalities, written in C, multitasking, and with loadable device drivers, networking, most of the modern OS expectations, really. Unfortunately compatibility with real mode DOS programs directly manipulating the hardware was spotty and the 286 support was broken on early Intel 286 revisions. This would eventually become DR DOS. It was the only DOS-like OS that actually supported multitasking (some) real mode programs in protected mode on the 286. Quite a tricky feat.
Between spotty compatibility, Microsoft trying to keep Windows from running on DR DOS, a high price and tiny msrketshare it never got mass adoption. Did see some use in business. The PC would not get such features in the mainstream until Windows in the 90s.