Mass and volume efficient solar arrays are sought by NASA, DoD and commercial space to enable high power missions from 20-50 kW in the near term and eventually up to 350 kW. Currently, the maximum power available from conventional solar arrays, for a given spacecraft, is limited by either the weight or stowage volume of the honeycomb panel substrates. Flexible substrate arrays can enable higher power spacecraft by improving specific power (W/kg) and specific volume (kW/m3) as well as improving the deployed natural frequency. Typical designs for flexible substrate array require a stiff boom mechanism to deploy the array and provide the deployed structure. Heritage flexible substrate arrays have used metallic slit-tube or coilable longeron booms. To be feasible, large, next-generation flexible substrate solar arrays require deployable booms that are more thermally stable than metallic slit-tubes (STEMs), and less expensive and lighter than coilable longeron booms (i.e. AstroMast). To address this need, CTD has developed the Stable Tubular Extendible Lock-Out Composite Boom (STELOC Boom). The STELOC Boom can provide stiffness equivalent to coilable longeron booms with a significantly reduced volume, mass and cost. The Phase I program demonstrated feasibility of the STELOC boom as the deployment actuator and primary structural component of a 15 kW solar array wing. The proposed Phase II program will advance the STELOC Boom to TRL 5 through the design, fabrication and testing of a flight-like Engineering Development Unit.