The 3rd Generation Partnership Project (3GPP), the standard development organization in charge of the 5G mobile system specifications, is currently developing the adaption of the NB-IoT protocol for non-terrestrial networks (NTN), referred to as NB-IoT NTN. The NB-IoT NTN protocol is expected to operate, among others, in LEO constellations. To that end, the NB-IoT protocol originally conceived for terrestrial access is being enhanced to address the specific characteristics of LEO satellite channels, including e.g. timing relationship enhancements in random access and scheduling processes to deal with the longer propagation delays of satellite channels and new Doppler precompensation mechanisms. Mobility management improvements to cope with moving satellite cells and noncontinuous operation of the service link are also being worked out, leading to e.g. the specification of new system information messages encoding satellite ephemerides data and specific tracking area management approaches for satellite access. The possibility to use the same protocol NB-IoT for both terrestrial and satellite access offers an unprecedented opportunity for massive IoT services, which will benefit from the global coverage provided by satellites. In this context, this paper describes the key design considerations and main system components of a NB-IoT NTN solution that is being developed for operation with CubeSat platforms in low-density LEO constellations. Results and discussion of technical challenges are provided in the areas of link budget, beam layout configurations, capacity and achievable service performance indicators.