Create an ISO based on the unpacked Win3.11 install files and mount it as D:\.
Install a CD-ROM driver into MS-DOS so that you can mount a CD as D:\ for example.But basically, a floppy disk boot is going to be used one way or another.
Or if you have a DOS boot floppy that has CD-ROM drivers built in, boot that and then mount an ISO containing all the unpacked MS-DOS floppy files.Install MS-DOS from floppy disk images (no CD/ISO based install is supported for MS-DOS).It will not be directly bootable, however. You could turn that into an ISO file using any ISO creation tool.
On this page I have an "unpacked" Windows 3.11 install folder (it's a single zip file that contains all files from all Win3.11 floppy disks). MS-DOS and Windows 3.1 were originally distributed as floppy disks, so no CD-based installation is supported. Note that two VMs can not be powered on at the same time if they have the same disk attached!
Windows XP or Linux) which does support guest additions.
Instead, download all the components and install it yourself. I'm no longer hosting the bundled tarball that contains all the files + a VirtualBox preinstalled disk image to clean up disk space on my server.
When you get a "Can't read file" error, you'll usually swap in the next numbered disk and hit enter. Windows 3.1 can still be installed from these images, it will just require more disk juggling. I got ahold of this version of Windows 3.1 from a CD image instead of floppies, so I had to convert them to floppy images myself, and not all the files fit on all the disks (there should only be 6 disk images but there's 7 in this tarball). The version of Windows in this tarball is 3.1 - not Windows for Workgroups 3.11 (for that, download it separately from my MS-DOS page).