We propose and demonstrate a wireless communications system wherein multiple Unmanned Aerial Vehicles (UAVs) collaborate for Distributed Transmit Beamforming (DTBF) without feedback from the target receiver. This system can extend the collaborative downlink communications range without a single point of failure and allow the target to be agnostic to aspects of beamforming. However, it faces considerable challenges due to channel variations from the UAVs hovering and the need for timely and accurate synchronization. We devise an intra-network protocol using Gold codes for simultaneous channel sounding and fast frequency offset compensation among the UAVs. The received signal model is developed and compared to the case where receiver feedback is available. Statistical channel models for UAV hovering and oscillator stability are experimentally derived and used to evaluate the coherence time and beamforming performance at 915, 2550, and 5900 MHz. A prototype was implemented using software-defined radios and used to conduct the first demonstrations of DTBF without feedback in a mobile environment. Our experiments with two DJI M100 UAVs achieved convergence in 200 ms with beamforming gains over 90% of the theoretical maximum and within 10% of our modeling predictions, validating the proposed design.