Detailed, large-scale 3D models of cities are important assets for many applications. While creating such models is difficult and time consuming, keeping them updated is even more challenging. In comparison, in many domains, crowd-sourcing of data is now an established process for expanding the scope or detail of data sets. In this paper, we describe the initial prototype implementation of Open3D, a crowd-sourcing platform for distributed curation of large-scale city models. We present an open architecture with interfaces that clearly separate model storage and indexing from viewing or editing. To support collaborative editing of extremely large models, we propose to use a modeling and model description paradigm that can integrate polygon-based modeling with parametric operations. We demonstrate the main concepts and prototype through an online city model that can be synchronously edited by multiple users, with live changes being propagated among clients. The main implementation consists of a set of web services, which support key functions such as model storage, locks for editing and spatial queries; a light-weight viewer based on the Cesium library, which runs on desktops and mobile devices; and a prototype editor, which clients can install to edit the models.