LAKE ORION TOWNSHIP, Mich. — In a nondescript supplier park in suburban Detroit, an electric vehicle startup backed by Amazon founder Jeff Bezos is building what it hopes will be America's newest automaker.

The facility is filled with dozens of prototypes, crash-tested vehicles, a crude lab vehicle skeleton adorned with wires and, most importantly, a busy "beta" assembly line that has been building electric vehicles since December for the startup, Slate Auto.

Slate is using the location — a stone's throw away from a massive General Motors assembly plant — to produce more than 70 vehicles for internal testing, certification and everything else a company needs to prepare to produce and sell vehicles in the United States.

The beta production line features roughly a do

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