There will be two phases to this study. The lead-in phase will evaluate the safety, pharmacokinetics, and define the maximum tolerated dose (MTD) of momelotinib combined with nab-paclitaxel and gemcitabine in adults with previously untreated metastatic pancreatic ductal adenocarcinoma. The randomized treatment phase will evaluate the efficacy, safety, and tolerability of nab-paclitaxel and gemcitabine combined with either momelotinib administered at the MTD or placebo in adults with previously untreated metastatic pancreatic ductal adenocarcinoma as measured by improvement in overall survival (OS), progression free survival (PFS), and overall response rate (ORR). Participants will continue study treatment until disease progression, unacceptable toxicity, consent withdrawal, or participant’s refusal of treatment. Following treatment, participants will be followed for safety for 30 days and for survival approximately every 3 months for up to 5 years.