Somerville Mayor Joseph Curtatone’s proposal for “Amazon On The Orange Line” is adding a last-minute wrinkle to Greater Boston’s attempts to lure the online commerce giant’s second headquarters.

Somerville’s recruitment pitch focuses on sites near the MBTA’s second-most-traveled subway line including parcels in Cambridge, Somerville and Boston’s North Station. Boston is finalizing its own short list of preferred sites within the city to present to Amazon by next Thursday’s deadline.

Curtatone’s proposal mentions six properties that could meet Amazon’s requirement for approximately 8 million square feet of office space:

• Federal Realty Investment Trust’s Assembly Row in Somerville, which is marketing 1.5 million square feet of build-to-suit office and lab space;
• Boston Properties and Delaware North Cos.’ Hub On Causeway development at Boston’s North Station, which has 175,000 square feet of office space in its first phase now under construction as well as approvals to build a 21-story, 520,000-square-foot office tower;
• DivcoWest’s NorthPoint on the Cambridge-Somerville border, which is approved for 2.1 million square feet of office and lab space;
• US2 Assoc.’s Union Square redevelopment, which has approvals for 1.4 million square feet of commercial space;
• Boynton Yards in Somerville, with potential for over 800,000 square feet of commercial development near the Green Line Extension’s future Union Square station;
• 50 Middlesex St. in Somerville, a 9-acre site next to Assembly Row that’s under agreement to Novaya Real Estate Ventures, Cresset Partners and Eastern Real Estate.

A summary of Curtatone’s proposal does not specify financial incentives for Amazon, but notes that “in order to justify this investment, the economic benefits created by a new Amazon headquarters must be shared amongst many parties.”

Boston Mayor Marty Walsh said Thursday the city is still narrowing down its menu of options to submit to Amazon, which is looking for approximately 8 million square feet of office space, including a first phase of 500,000 square feet in 2019.

“Suffolk Downs is the one we’ve put a lot of emphasis on with the proximity to the airport, and the land is available. You have two train stations there and one owner, and you have the city of Revere and Boston pretty much aligned in what could go there,” Walsh told Banker & Tradesman.

Boston developer Thomas O’Brien’s HYM Investment Group in May acquired the Suffolk Downs racetrack property for $155 million. O’Brien initially said the site could be developed into an Assembly Row-like complex including multifamily housing, office space and life science labs.

Other sites still in the mix include the Beacon Yards former rail yard in Allston and multiple sites near South Station and the Seaport District, Walsh said.

Gov. Charlie Baker is taking a pass on narrowing down the growing list of sites in Massachusetts, as mayors and economic development officials across the state tout the advantages of nearly 20 sites.

Boston is viewed as a strong candidate because of its concentration of tech employees, extensive public transit system and proximity to Logan International Airport.

Economic researchers Moody’s Analytics ranked Boston ninth on its list of most likely Amazon landing spots, with Austin, Texas taking the top position. The economic researchers ranked cities based upon their business environment, human capital, costs, quality of life and transportation. Boston ranked third in the business environment but next to last in cost of living, ahead only of the New York metro region.

Boston came in second in The New York Times’ analysis of potential Amazon sites; Denver took the top spot. Despite ticking many of the boxes on Amazon’s wish list, “it’s hard to imagine where the Boston region would find the room for a company that will ultimately want up to eight million square feet of office space,” the Times concluded.

“I don’t know what Amazon is looking for and I think they are looking at these proposals to see who’s being creative and what might work for them,” Walsh said. “So their idea probably initially is a campus like they have in Seattle, but after they see some of the proposals they might think differently. We’re just trying to get to the next round.”

Curtatone notified Cambridge officials of his intent to include NorthPoint in its submission, Cambridge spokesman Lee Gianetti said. Nearly 39 of the site’s 45 acres are located in Cambridge, but the city is not submitting its own proposal to Amazon.

“Cambridge is supportive of the state’s efforts to recruit Amazon to the commonwealth and excited to assist with their efforts,” Gianetti said.