This paper describes a methodology for shape optimization of the envelope of an airship for long endurance missions at stratospheric altitudes. Gertler Series-58 shape generation scheme is selected for optimization studies, in which the envelope shape is driven by four shape parameters and fineness ratio. The values of these parameters are obtained by satisfying the condition of airship geometry. Effect of operational parameters viz. location, altitude, wind etc. on sizing of airship are discussed. For multidisciplinary design optimization (MDO) A composite objective function is formulated which incorporates the value of envelope volumetric drag coefficient (CDV), circumferential hoop stress (σhoop) on the envelope, area of solar array and envelope surface area. The optimization is carried out using an open-source implementation of a robust stochastic algorithm, viz., Genetic Algorithm.