Fleets of UAVs will be deployed in near future in reliability and safety critical applications (e.g. for smart cities). To satisfy the stringent level of criticality, each UAV in the fleet must trust the other UAVs with which it communicates to get assurance of the trustworthiness in information received and to be sure not to disclose information to an unauthorized party. In addition, to be protected against an attacker willing to eavesdrop and/or modify the exchanged data, the communication channel needs to be secured, i.e. it has to provide confidentiality and integrity of exchanges. The work presented here is based on our previous research which concluded that it is required that each UAV includes a Secure Element (which we called ARFSSD standing for Active Radio Frequency Smart Secure Device) to withstand an adversary with a high attack potential. In this paper, we propose a secure and trusted channel protocol that satisfies the stated security and operational requirements for a UAV-to-UAV communication protocol. This protocol supports three main objectives: (1) it provides the assurance that all communicating entities can trust each other and can trust their internal (secure) software and hardware states; (2) it establishes a fair key exchange process between all communicating entities so as to provide a secure channel; (3) it is efficient for both the initial start-up of the network and when resuming a session after a cold and/or warm restart of a UAV. The proposed protocol is formally verified using CasperFDR and AVISPA.