I’m not sure why you would want a flash drive style device that is capable of destroying some of the components inside a PC. The only use I can think of is vandalism at schools or work, which is what at least a few of the commenters on a YouTube video posted by Thomas Kim seem to be interested in. Kim posted a video showing the steps needed to build a DIY USB Killer that blasts voltage through the USB port of a PC to make it shut down.
The build requires a disposable camera flash circuit with power appearing to come via a AAA battery. The parts of that flash circuit needed for the project includes the transistor, transformer, resistor, diode, and capacitor. These components are removed using a soldering iron.
These components were then grafted onto some sort of battery holder. The USB connector needs only the white and green wires for the device to function. Copious amounts of hot glue were used to secure the components to the back of the battery holder and keep the wires in place.
When complete the USB killer produces 300V DC essentially making a taser with more than enough power to fry a computer. Exactly what components are fried inside the PC are unknown. The video offers no look at the PC internals after the device is discharged. Speculation in the comments suggests that most of the main PC components would be unaffected and that the death of the PC is likely related to a mainboard failure.