Microsoft announced on Wednesday that it has completed the development of Windows 8, its next OS for PCs and tablets and the successor to Windows 7, which is the best-selling OS of all time. It’s unclear why the company waited so long, however: The final build was ready days earlier.

Microsoft previously announced that it planned to finalize the system in the first week of August and deliver it to customers via software upgrades and new PC purchases on October 26.

“A short while ago, we started releasing Windows 8 to PC OEM and manufacturing partners,” Microsoft’s Steven Sinofsky writes in a post to the Building Windows 8 Blog. “This means our next milestone will be the availability of exciting new models of PCs loaded with Windows 8 and online availability of Windows 8 on October 26, 2012.”

Windows 8 represents a radical departure from previous Windows versions and is arguably the most dramatic upgrade Microsoft has yet developed. The system is essentially a brand-new mobile platform that has been melded onto the traditional Windows desktop, giving users what Microsoft calls a “no compromises” experience that blends the best of mobile with the best of Windows.

Windows 8 will be available to users of both traditional, Intel-style PCs and new, ARM-based devices. The versions of Windows 8 that run on the latter are collectively branded as Windows RT, and although this system offers a lot of compatibility with Windows 8, it can't run legacy Windows desktop applications.

Windows 8 will also trigger an avalanche of new PC and device types. In addition to traditional tower PCs and laptop computers, users can expect hundreds of new designs in the fall, including touch-capable Ultrabooks, all-in-one computers, and hybrid systems of all kinds. Tablet devices that look like the iPad but offer the full Windows experience will likely be common, and Microsoft has separately announced Windows 8-specific mouse devices and keyboards that it will sell to upgraders.

Microsoft did not announce any further pricing information for Windows 8, although it is offering current PC buyers a $15 upgrade to Windows 8 Pro and will offer a temporary $40 upgrade to Windows 8 Pro for users who want to upgrade older PCs running Windows XP, Windows Vista, or Windows 7. No word on Microsoft’s Surface devices, either, though the Windows RT-based version of Surface will ship at the same time as Windows 8/Windows RT.

Perhaps because of the radical changes in this OS, there are many questions surrounding Windows 8. And Microsoft hasn’t been very forthcoming about some of the more pressing questions. One key question is how it plans to upgrade the system going forward, but my sources tell me that Microsoft won't wait three years to deliver a Windows 9 release. Instead, Windows 8 will be updated on some kind of a yearly schedule.

My sources also tell me that Windows 8 was complete for about two weeks before Microsoft officially declared RTM, and the final build was actually delivered internally and to key partners days ago. (The final build was compiled on July 25.) Windows executives had spent much of the previous week debating internal matters such as what the final build number would be: Microsoft routinely and arbitrarily increments the build to meet some internal desire to be witty, and Windows 8 was no different. After moving through various 85xx build numbers late in development, Microsoft decided that the final build number would be—wait for it—8888. But a showstopper bug necessitated incrementing the build again, to 9200.

With Windows 8 now complete, my latest book, Windows 8 Secrets, can be sent off to the printers, and I’ll turn my attention to updating my Windows 8 Feature Focus articles on the SuperSite for Windows and providing a final, multi-part review. In the meantime, you can expect to see Windows 8 become available through the following channels:

MSDN and TechNet: Subscribers to these services will receive Windows 8 RTM on August 15.

Volume licensing customers with Software Assurance: Customers on Microsoft’s Software Assurance (SA) program will receive Windows 8 on August 16.

Microsoft Partner Network: August 16.

Microsoft Action Pack Providers: August 20.

Volume license customers without SA: September 1.