The company made the announcement at its Google I/O 2016 annual developer conference, where it also said it planned to have a consumer model of Ara on sale in 2017.
The modular phone is designed to have a longer lifespan than current products on the market, because its hardware can be easily changed and updated.
Third-party designers are now being invited to contribute hardware modules to the platform, using the developer edition of the phone that is due to ship in autumn of 2016.
The video shows how Project Ara's blocks can be easily unclipped from the phone's base – which has room for six modules – and swapped for alternative ones, like new camera lenses.
These modules can be slid into any slot on the phone, which is powered by new Greybus software that promises "instantaneous connections, power efficiency and data-transfer rates of up to 11.9Gbps".
The Ara frame contains the phone's core, immovable functions – CPU, GPU, antennas, sensors, battery and display – creating more room for developers to include hardware in modules.
These modules are held in place with latches and connectors and have been designed to remain compatible with future generations of the phone.
"We are looking to module makers to create technology never before seen on smartphones," said Google.
"We're hard at work on an open marketplace to connect the next generation of ideas with the broader Ara community. Google is looking for beloved brands and forward-thinking developers to join us."
Project Ara started out life as a concept for a modular phone designed by Eindhoven graduate Dave Hakkens.
The designer teamed up with then Google-owned communications company Motorola in 2013 to bring the idea to life, with Google unveiling a fresh prototype of the modular smartphone at the start of 2015.
The possibility of external developers being able to contribute their own modules spiked designers' interests, with San Francisco studio Lapka creating a set of seven conceptual health blocks that could fit onto the phone to track everything from air quality to heart rate.
The video showing the finalised Project Ara
According to the Project Ara site, Google plans to ship developer editions of the smartphone from autumn 2016, with a consumer version to follow in 2017.