Sort of. Digital Research had two teams, in different countries, that developed the product that became DR-DOS in two stages. The company that purchased DR-DOS only claimed in court that the second team had done no reverse-engineering of MS-DOS. However, it also said that there was code written by the first team in DR-DOS, so the final product was not entirely the work of the clean room.
The DR side of the story came out in Caldera v. Microsoft. That was an interesting story in itself: Novell purchased Digital Research in 1991, and then, according to the judgment in a lawsuit Novell filed against Caldera,
Novell owned the source code for DR DOS, a computer operating system that was the target of anticompetitive practices by Microsoft in the early 1990s. Novell's board of directors worried that, if they brought suit against Microsoft in a private antitrust action, Microsoft would retaliate with further unfair practices that could neutralize the value of any antitrust recovery. At the same time, however, Novell's board of directors recognized that Novell's shareholders would not permit Novell to simply walk away from such a significant cause of action and potential recovery.
Accordingly, Novell entered into negotiations with Caldera, Inc., the predecessor in interest to Canopy, to sell DR DOS to Canopy. The main purposes of this sale were to obligate Canopy to bring suit against Microsoft, to allow Novell to share in the recovery, and at the same time to obfuscate Novell's role in the action against Microsoft. Novell insisted that its role be completely undetectable to avoid retaliation from Microsoft.
To accomplish this, Novell and Canopy executed two separate documents: the first was a contract of sale, obligating Canopy to pay $400,000 for rights to the source code; the second was a temporary license obligating Canopy to pay $600,000 in license fees and "royalties." The royalties included provisions for payment to Novell of a percentage of any recoveries from lawsuits.
This all came out when Novell sued Caldera (by then part of the Canopy Group) for not giving them a big enough cut of their legal settlement it received from Microsoft. Novell would later file its own lawsuit against Microsoft, alleging antitrust violations against its suite of office software.
But in that Caldera v. Microsoft lawsuit, which alleged that Microsoft broke the law to stop DR-DOS, the former boss of Digital Research gave its version of the history of DR-DOS. (Emphasis added.)
A dynamic new leader, Dick Williams, took the helm DRI on January 5, 1987. Prior to Williams’ arrival, DRI had been approached by several OEMs who, unhappy with MS-DOS and Microsoft’s control of the operating system market, had urged DRI to develop a competing product. Within 30 days of taking control of the company, Williams had begun the development of DR DOS.
John Constant, a DRI engineer in Hungerford, England, began development of
DR DOS using an existing DRI product, Concurrent DOS. Constant did no “reverse engineering” of MS-DOS. Rather, he built upon the DOS compatibility DRI had already developed in Concurrent DOS.
In other documents it filed in the suit, Caldera provided some more detail. (The same document has some information relevant to your other recent question about sales of DR-DOS in Japan.)
Although development of DR DOS was undertaken at DRI’s European Development Center (the “EDC”) in Hungerford, England, significant portions of the code included in the initial version of DR DOS came directly from Concurrent DOS, which was developed by DRI engineers at DRI’s facilities in Monterey, California. EDC engineers developed DR DOS for DRI as a work-for-hire, and DRI has at all times owned the product. [...] (“the EDC was the only facility outside of the United States that ever [(illegible)]-aged in software development [for DRI] other than for product translation”)
Notably, Caldera’s lawyers never claim that Digital Research’s Concurrent DOS project had not reverse-engineered MS-DOS, only that John Constant did not do so on when turning their code into DR-DOS, in his separate room on another continent. The lawsuit is, in fact, largely about undocumented interfaces and data structures needed for 100% compatibility, which it would have been impossible to duplicate without reverse-engineering. What keeps this from being a clean-room implementation is that Caldera admits DR-DOS incorporates some code written in Monterey.
However, just as significantly, Microsoft did not countersue or raise a copyright claim against DR-DOS. Indeed, when it came out, Microsoft attempted to purchase it.
The lawsuit also revealed, among other things, that Steve Ballmer of Microsoft approached Digital Research in December 1988. “Steve Ballmer had a promising discussion with DRI re: them licensing MS-DOS
vs. competing with us.” Joachim Kempin, MS’ Microsoft’s head of OEM sales, testified that Microsoft wanted to license DR-DOS, and give DR a license to distribute the new MS-DOS. Caldera claimed that DR did not want an ongoing relationship with MS, asked for a one-time cash payment instead, and MS refused.