Britain, the Netherlands, Finland and Poland are making "significant progress" on the new Multilateral Defence Mechanism and aim to establish it by 2027, the countries have said.
Defence financing is set to be a key topic at this week's NATO summit in Ankara, Türkiye.
The MDM is one of several competing initiatives aimed at channelling more private cash into rearming nations.
"We have benefited from the support of a broader group of allies in developing the technical details of this model," the four nations said in their joint statement on Monday.
They said they would work to expand the coalition of nations involved in the project and would move on to the next phase of developing the mechanism with countries that have declared their participation in the autumn.
Growing number of defence financing initiatives
Another key project in the increasingly crowded field of defence financing initiatives is the Defence, Security and Resilience Bank.
Canada is aiming to announce around 10 founding nations for the DSRB at the NATO summit.
The Polish finance ministry did not immediately respond to an emailed question about whether it was in talks to join the DSRB.
Britain has ruled out participation in the DSRB, opting to pursue the MDM project instead.
