This study is open to adult men with cystic fibrosis and adult women with cystic fibrosis who cannot have children. People with cystic fibrosis can join if they are not eligible to receive cystic fibrosis transmembrane conductance regulator modulator therapy (CFTR-MT). The purpose of this study is to find out how well a medicine called BI 3720931 is tolerated and whether it improves lung function in people with cystic fibrosis. In this study, BI 3720931 is given to humans for the first time. This study has two phases. In Phase 1, participants are put in one of 3 groups, one group after the other. Each group gets a different dose of BI 3720931. Group 1 starts with the lowest dose, followed by group 2 with the middle and group 3 with the high dose. In Phase 2, participants are put into 3 groups by chance, but at the same time. 2 groups get different doses of BI 3720931, and 1 group gets placebo. All study participants get only 1 dose of BI 3720931 or placebo and they use a special inhaler to take the study medicine. The placebo inhaler looks like the BI 3720931 inhaler but does not contain any medicine. During the study, participants continue taking their usual medicines. Doctors closely monitor participants' health at the study site for the first 3 days after receiving BI 3720931. Participants visit their doctors regularly thereafter. The doctors check the health of the participants and note any health problems that could have been caused by BI 3720931. Study participants regularly do a standard lung function test to measure how well their lungs are working. Participants, in either Phase 1 or Phase 2, are in the study for 7 months. After completion of this study, participants will take part in a long-term follow-up study (1504-0003).