Model Based Systems Engineering (MBSE) approaches are rapidly spreading among organizations and industries due to all their claimed benefits over traditional document- based approaches. Benefits include for instance enhanced design quality of systems, clearer development of system requirements and specifications and improved communications within the design teams. Currently, MBSE methods and tools are mainly employed to successfully develop complex systems, such as aircraft or its components. However, this paper proposes to adopt MBSE also in the design of development systems, which aim to design complex systems. In particular, this paper focuses on the first activities of a typical Systems Engineering Product Development process: identification of system stakeholders, collection of their needs and development of system requirements. The main outcome delivered from this paper is an architectural framework, i.e. a guideline for the modeling of complex systems. More specifically, the architectural framework is still under development, and hence the current version focuses on the modeling of stakeholders, needs and requirements of complex systems. The focus of the proposed architectural framework is on the agility for the definition phases of complex systems. In other words, it is developed to streamline, improve and accelerate the definition and modeling of complex systems. Details of the architectural framework including the means to represent all the system information are provided. In addition, the architectural framework for the development of complex systems is supported by an MBSE development system, currently being addressed in the EU- funded research project AGILE 4.0. The MBSE development system is presented in this paper together with an example of its application for the definition of complex systems: an horizontal tail plane for a regional jet aircraft, designed and manufactured within an aeronautical supply chain consisting of different companies.