The Friday morning announcement comes just days after reports began circulating that GM would join Amazon in the new round of financing. But a well-placed source involved in the negotiations told CNBC that "you don't find all the presents under the Christmas tree," suggesting that another development is in the works, one that could come at or before the New York Auto Show in April where Rivian is scheduled to hold a news conference.

The announcement comes less than three months after Rivian revealed its first two products, an all-electric pickup and SUV that, some analysts have said, could pose a direct challenge to both established truck manufacturers like General Motors and Ford , as well as upstart Tesla which is developing its own battery-electric pickup.

"This investment is an important milestone for Rivian and the shift to sustainable mobility," said RJ Scaringe, Rivian founder and CEO. "Beyond simply eliminating compromises that exist around performance, capability and efficiency, we are working to drive innovation across the entire customer experience. Delivering on this vision requires the right partners, and we are excited to have Amazon with us on our journey to create products, technology and experiences that reset expectations of what is possible."

The announcement did not detail how much Amazon specifically will invest in Rivian. For the ecommerce giant, the move follows barely a week after it joined in with venture capital firm Sequoia, as well as Lightspeed Venture Partners, Shell Ventures and others, to take a stake in Aurora. That deal put a roughly $2.5 billion valuation on Aurora, an autonomous vehicle technology start-up run by Waymo and Tesla alumni.

Exactly why Amazon would want to make such moves remains hazy, though it follows steps taken by other high-tech firms, such as Alphabet and Apple to explore opportunities in electrified and autonomous mobility.

Amazon, in particular, has been looking for ways to change up its delivery methods which currently rely on traditional partners such as the USPS and UPS. Some industry analysts have speculated that Amazon could be interested in Rivian's platform for developing vehicles for its massive global logistics needs.

"We're inspired by Rivian's vision for the future of electric transportation," said Jeff Wilke, Amazon's worldwide consumer CEO. "RJ has built an impressive organization, with a product portfolio and technology to match. We're thrilled to invest in such an innovative company."

The round included investments from Rivian's existing shareholders. The start-up will remain an independent company. Rivian currently has more than 750 employees located in several locations, including Plymouth, Michigan; San Jose and Irvine, California; Normal, Illinois; and Surrey, England.

The fact that GM was not included in Friday morning's announcement does not mean the Detroit automaker won't subsequently take a stake in Rivian. GM issued a terse release earlier this week that notably sidestepped the question, only stating that "We admire Rivian's contribution to a future of zero-emissions and an all-electric future."

Until recently, few expected to see a competitive, battery-powered electric pickup because of the hefty demands owners place on those vehicles. But views have begun to shift as Tesla prepares to bring a truck to market and as analysts and potential buyers have gotten a look at the R1T pickup that Rivian unveiled at the Los Angeles Auto Show last November – along with the R1S sport-utility vehicle.

Rivian plans to launch its R1T pickup truck and R1S sport utility in the U.S. in 2020, and begin introducing them overseas in 2021. The company has modeled both vehicles on what it calls a "skateboard" platform, which it says it flexible enough to accommodate several different vehicle body styles.

Both, at least initially, will feature 180 kilowatt-hour battery packs, almost twice the size of the largest pack currently offered by Tesla. That will be more than enough to deliver a range of 400 miles under optimal conditions, according to Rivian CEO RJ Scaringe.