William Kaven Architecture wants to design the tallest building in the Pacific Northwest. And it wants to put it on Portland's Pearl District Post Office blocks, the site the city offered to Amazon for its second headquarters.

The concept, released by the Portland firm on Monday, proposed two skyscrapers, one of which would rise 970 feet. Together, the high rises would provide about 5 million square feet for retail, office, hotel rooms, apartments or condos.

According to the announcement, the two buildings would be linked by "a glass-enclosed botanical bridge spanning 236 feet across the North Park Blocks some 680 feet in the air, providing dramatic aerial views of the entire city."

The proposal would also be able to accommodate a transportation hub for high-speed rail or a Hyperloop, the news release said.

But proposed height limits for the site would allow buildings as tall as 400 feet.

In an email to The Oregonian/OregonLive, Daniel Kaven, a partner with the architecture firm, acknowledged the constraints of the height limits. But he said the city, which is considering zoning changes in its Central City 2035 plan, should think big.

"City Council is able to amend this if needed/desired," he wrote. "It is our belief that there should not be a limit on the height and that vertical development on this scale is necessary."

In the firm's announcement, it said its proposed towers would be large enough to serve as a headquarters for a Fortune 100 company like as Amazon. In early September, the e-commerce giant announced it was in the market for a second company headquarters, one that could accommodate 50,000 employees, and that it was prepared to invest $5 billion in construction alone. Last month, Amazon said it said it received 238 proposals from municipalities throughout North America.

But Portland, so close to the company's Seattle headquarters, is viewed a long shot for Amazon's second home base. Last week, Prosper Portland, the city's urban renewal agency, called on developers to submit applications to craft a master plan for the Post Office site and the surrounding blocks, dubbed the "Broadway Corridor."

The city hopes to implement this master plan regardless of who the tenants might be.

William Kaven intends to submit a formal proposal to Prosper Portland early next year.

-- Anna Marum

amarum@oregonian.com

503-294-5911

@annamarum