For those who are not aware, even though Windows was not released until 1985, it actually made its first appearance at the Fall 1983 COMDEX show, and later the December 1983 issue of Byte magazine.

"Developer Release 5" as it is known is so far the earliest known surviving build of Windows to exist, having been released in late 1984.
It is still very primitive, required either DOS 2.x or 3.0 on a PC/XT to run (although allegedly it will run on later versions as it doesn't do a version check), a MS Serial or Bus mouse, and either monochrome CGA or Hercules video. A colour CGA monitor will not work!
It comes with a few test programs and source code. In order to install, you need to copy the INSTALL.BAT file to C:\ and run it from there.

These screenshots are taken in CGA mode, with line doubling turned on, so they may look strange.

Here is the MS-DOS Executive in DR5. Notice that it installs itself into C:\WINDOWS\TEST. There is also a C:\WINDOWS\BIN directory containing some development utilities
Also note the lack of a menu on the system box (the upper left button), and the lack of any kind of About dialog box.
Executables are NOT binary-compatible with final Windows 1.0 ones, and it appears program resources are stored external to the actual program (that is what all those .RES files are).

A BIN folder is kind of odd to see on Windows as that is traditionally a UNIX archaism, but it's worth noting that Microsoft was heavily into UNIX at the time.
Also interesting is that you do not directly start the Windows executable from DOS with the familiar "win" command. You are expected to run a batch file called "mswin.bat" which runs all the necessary startup programs for you.
Other than that, the fit and finish is mostly similar to the final release.

Here is a dialog box, which has a significantly different appearance than the final release.
Note the lack of any real buttons; the menu bar actually acts as the buttons!

Here is a sample program running, plus tiled windows to give a demonstration of those. Overlapping is not allowed, probably so Microsoft wouldn't get sued by Apple.

In practice a lot of the program for DR5 don't have dialog boxes; they output their messages in a bar near the top of the window.

Screenshots don't really convey how buggy and unstable DR5 really is. I ran into several unexpected glitches and some cases where running a program either hard-locked the computer or hosed the hard drive, or both.
In particular, I was never able to get Notepad to run. Also, on every reboot, the system path got trashed and I would have to manually re-enter it. Most times Windows would refuse to load again after that, complaining of some random thing failing to load.
Under DOS 2.1 I had a lot of problems with corrupt icons; these went away when I switched to DOS 3.0. Not sure why.