Following the withdrawal of its B-707 "La Reina del Espectro" and the failed attempt to replace it with a Gulfstream G550 due to budgetary constraints, the Spanish Air Force has seen a major decline in its ability to collect and analyze intelligence from electromagnetic signals. This type of intelligence, known as ROEM (Reconnaissance of Electromagnetic Origin), is currently limited to the SOAR payload, provided by American company L3Harris, which is used on MALE MQ-9A Reaper drones. To address this gap, Spain has launched the Santiago II program, aimed at restoring its electronic intelligence capabilities. In 2023, Indra, a defense electronics group, proposed to the Spanish Ministry of Defense the installation of "in-house" sensors on a Challenger 350 business jet, produced by Canadian manufacturer Bombardier, to convert it into an airborne electronic intelligence system. The system would be capable of intercepting communications (COMINT) and detecting radar presence. However, in December, Madrid instructed Indra and Airbus Defence & Space to conduct a study to identify the most suitable platform and sensors for a national solution based on three aircraft capable of detecting, classifying, and identifying targets of interest. Funded with 16 million euros, the project was expected to last eighteen months. Although the study is still ongoing, the platform choice has already been made. According to the official magazine Revista de Aeronáutica y Astronáutica, relayed by the specialized website InfoDefensa, the Spanish Air Force has selected the Airbus A321 as the platform. The specific version of the aircraft has not been specified. Four models are under consideration: the A321ceo (for Current Engine Option), the A321neo (with 7,400 km of autonomy), the A321LR (same, but with additional fuel tanks), and the A321XLR, capable of traveling 8,700 km. The latter model was chosen by the French Direction générale de l'armement (DGA) for the development of the future maritime patrol aircraft of the French Navy, making it the favorite for the Santiago II program. Airbus emphasized its "extensive experience in converting commercial aircraft into military aircraft," citing the A330 MRTT as an example. InfoDefensa notes that the main advantage of a platform the size of the A321 lies in the space available for the installation of the mission system, as an electronic intelligence aircraft requires more than just external antennas: it requires receivers, processors, storage systems, communication links, operator stations, navigation equipment, self-protection systems, and a mission architecture capable of processing and correlating large volumes of information. According to Spanish budget documents, the cost of these three future A321 "SIGINT" (Signal Intelligence) aircraft is expected to amount to 436 million euros.