XPS 13 Developers edition (new one 2019) won't boot from USB

Is your USB stick and whatever OS you're trying to boot from it set up for UEFI booting rather than Legacy BIOS booting?  Apparently some newer Dell systems no longer support Legacy BIOS booting.  Initially that was disabled for internal storage but still allowed for USB devices if needed, but it seems it's being eliminated entirely -- possibly in preparation for the fact that Intel has announced that their CPUs will no longer support Legacy BIOS booting in 2021 or so.

In any case, AHCI vs. RAID won't affect USB booting, although RAID mode will prevent your system's internal storage from being visible in Linux.  You'll need to use AHCI mode.

Assuming it's a UEFI Parititoned USB Key, you need to turn off SecureBoot and also TPM. 

Also, do NOT boot the USB Key on Legacy Boot Mode.
Legacy Boot mode and UEFI mode don't work together.

If you manage to boot under Legacy Boot Mode, your UEFI Ubuntu installation will likely fail at the end when it tries to write boot partition information. 

It certainly did for me during my Pop/OS (variation of Ubuntu) dual boot installation and I was pulling my hair out for 2 days, until I figured out I had to disable TPM as well to get the USB boot working through UEFI.

The new one doesn't have the Legacy option, unless it's buried somewhere. I was not able to find it.

I created the bootable USB drive using DD and using RUFUS. My most recent attempt was using RUFUS which creates EUFI bootable drive.

Even turning secure boot and TPM off, I still only get the Dell splash screen and it will not boot off of the USB drive.

It's pretty disappointing how much effort it is taking in order to install an operating system.

Ok, Rufus should definitely work as long as you choose to make a GPT disk for UEFI booting, since Rufus also supports creating Legacy BIOS boot media.  How is the flash drive connected to the system?  If it's through a dock or hub, can you try connecting it directly, or through just a simple USB-C to USB-A adapter dongle if necessary?  Sometimes having "active" devices in the USB data path can create problems with bootable devices.  Also, have you tried a different flash drive?  I've seen some systems that simply won't boot from certain flash drives.  I'm not sure why, but it does happen.

Legacy Option ROMs isn’t needed. That actually enables Legacy BIOS boot options. The difference between UEFI + Legacy Option ROMs and full Legacy mode is that the latter allows ONLY Legacy boot options, with UEFI completely disabled.

Here are my BIOS Settings:

• Settings
  • Boot Sequence
    • UEFI : ON
  • Advanced Boot Options
    • Enable Legacy Option Roms : ON (Not sure if this is needed)
• System Config
  • USB Config
    • Enable USB Boot Support : ON
    • SATA Operation : AHCI
  • Security
    • TPM 2.0 Security
      • TPM ON : OFF
  • Secure Boot
    • Secure Boot Enable : OFF

I also used Etcher instead of Rufus. Give that a go.
https://www.balena.io/etcher/

I've had problems with Rufus is the past. It could be a case by case thing with some USB keys working well with Rufus and others don't

Have exact the same issue. I tried different USB Sticks, rufus and etcher - but Ubuntu 19 wont boot. I tried ubuntu 14, and that worked perfectly. I also tried a SD card, which worked perfectly on other laptops - but the XPS 7390 will not boot and stuck at the dell logo.

Any suggestions? TPM disabled, UEFI enabled, switched to AHCI, Secure Boot disabled.

okay, solved. The solution is simple - only Ubuntu 18.04 has GPT extended boot loader definition in the iso. PopOS / Fedora does not have. XPS 13 7390 confirmed to install Ubuntu.

Have you checked whether the bootable media is recognized by all USB Type-C ports? Incidentally, under 'POST Behavior', change "Fastboot" to "Thorough", which prevents intermittent boot failures.

Have. Weird thing is really that only 18.04 boot and get installed, any other distro does not boot from USB. Update to the newest bios, different USB Sticks tested again / nothing. Stuck at the Dell logo. On the same UCB-C port Ubunutu 18.04. boot without issues.

Thanks for testing that out and posting your results but it's not really a solution to the problem. I want to install a different OS other than Ubuntu LTS.

I was only able to boot Fedora 31 and Ubuntu 19.10 (and of course the dell Ubuntu Image).

I suspect there might be a minimum kernel version that will successfully boot on these. 

Mine is the XPS 7390 non 2in1

https://github.com/NixOS/nixpkgs/issues/72936

Its probably a grub bug

It isn't on a dock. It's directly to the computer. I've tried 3 different USB A using a dongle and one USB C drive that I bought specifically for this. It is a Samsung drive.

The drive vendors that I've used are

Samsung USB-C

Samsung USB-A

Sandisk USB-A

Kingston USB-A

Good find! The descriptions in the bug reports sounds exactly like what I'm experiencing.