SEATTLE, Jan 30 (Reuters) - Nikola Motor Company unveiled plans on Tuesday for a $1 billion manufacturing hub for its hydrogen-fueled semi-trucks, as the company seeks to expand in the nascent market for alternative-fuel freight trucking.

The 500-acre (202.34 hectare) Phoenix-area facility would be one of the largest stand-alone manufacturing facilities for next-generation semis, Nikola said, and is the latest sign of investor confidence that alternative-fuel trucks can gain a toehold in the freight market alongside relatively low-cost, time-tested diesel trucks.

The announcement for the plant comes more than two months after Tesla Inc unveiled plans for an all-electric semi-truck that would be in production in 2019, though it has not said where the truck will be built.

It also comes as companies like Navistar International Corp and Volkswagen AG VOWG-P.DE are working to launch electric medium-duty trucks by late 2019, and Daimler AG has delivered the first of a smaller range of electric trucks to customers in New York.

Nikola, currently based in Salt Lake City, Utah, manufactures hydrogen-electric vehicles and hydrogen fueling stations, and has received pre-orders for more than 8,000 of its semi trucks, it said.

Nikola said it expects construction in 2019 with the first trucks rolling off an assembly line in 2021.

The company declined to provide details of an incentive package it received from Arizona, but said the costs for developing the $1 billion facility would be shared between Nikola and the state. (Reporting by Eric M. Johnson in Seattle; Editing by Tom Brown)