The Industrial Internet of Things (IIoT) is central to smart manufacturing, enabling real-time automation, data exchange, and system intelligence. However, the convergence of cyber-physical systems with legacy software and heterogeneous architectures introduces significant security challenges. This paper explores how software engineering principles can be strategically employed to enhance IIoT security by integrating threat modeling into the development lifecycle. In this study, we review classic models such as STRIDE, DREAD, and STPA-Sec, and evaluate their effectiveness when applied at various phases of the Secure Software Development Life Cycle (SSDLC). STRIDE focuses on classifying security threats, DREAD helps score the severity of risks, and STPA-Sec provides a safety-oriented approach to identifying unsafe control actions in IIoT environments. Additionally, we propose a secure development process to embed continuous security assurance during IIoT software deployment. This research highlights design-driven security patterns, model-driven engineering strategies, and secure API development best practices. This paper aims to support developers and architects in designing scalable and threat-aware IIoT systems through the alignment of software engineering with IIoT-specific threat vectors.