The research work presented here focuses on the subject of transport aircraft design at the pre-design or conceptual level. The primary topics addressed are: (1) generation of a vast array of new quasi-analytical expressions to permit a conceptual treatment of commercial and business transport aircraft with adequate sensitivity for more advanced trade studies; (2) review and adoption of a method to predict stability and control characteristics (using the Mitchell method); (3) a study of the relative merits between various methods in facilitating an expedient and robust constrained multi-objective optimisation result within the context of traditional conceptual design problems (Genetic Algorithms and Nelder-Mead Simplex search); (4) creation of a software package as a new and unique conceptual tool that permits the generation of design proposals in an accurate yet expeditious manner; and, (5) practical demonstration of the new conceptual design software package by undertaking some actual aircraft design proposals. The design problem is addressed using mostly closed form solutions but transcendental expressions with much simplified numerical scheme algorithms have also been adopted for sake of accuracy. Various new models have been proposed for atmospheric properties, geometry, gas-turbine engine performance, low-speed and high-speed aerodynamic characteristics, minimum control speed limited balanced field estimation, asymmetric flight, and, en route performance characteristics including definition of operationally permissible speed schedules and flight techniques for payload-range/fixed sector profiles optimised in terms of maximum specific air range, minimum fuel, minimum time, minimum direct operating cost and maximum profit/return on investment. The work was extended further to include issues relating to the impact of vehicular attributes to pricing the market is willing to absorb. Useful information regarding how these individual computational elements of the methodology may be integrated for the purpose of constructing coherent modular sub-spaces and formulation of a basic inter-disciplinary coupling is also presented. The mathematical foundations derived in this work have lead to an array of tangible conclusions that aid the conceptual designer via implicit guidelines to achieve truly balanced design concepts. In an explicit demonstration of methodology effectiveness and relative simplicity, a software package called QCARD or Quick Conceptual Aircraft Research and Development was created in the MATLAB environment. The new software system was developed to assist the designer in predicting, visualising and optimising conceptual aircraft designs in a much more interactive and far-reaching manner than what is afforded with contemporary applications whilst emphasising speed and economy of effort. The methodology and software was employed for a 19 passenger turbofan commuter transport design using the cost effective Williams International FJ44-2 engines. To complement this, a fuselage stretch version of the baseline vehicle designed to accommodate 31-34 passengers was also undertaken utilising a growth version of the original FJ44 power plant. The minimum goal for both of these concepts was to afford unparalleled comfort through speed and spaciousness with a competitive edge against turboprops in terms of economics and field performance. The final design effort involved proposal of a Trans-Atlantic high-performance executive transport employing an unconventional Twin Oblique Lifting Surfaces, or, TOLS configuration. The intent here was to produce a new super-large business jet able to operate up to low supersonic speeds with field performance, en route fuel burn efficiency and cost comparable to that of contemporary business aircraft for this market segment. Keywords: aircraft, conceptual, design, specifications, atmosphere, geometry, weight, aerodyanmics, propulsion, operational performance, stability and control, constrained multi-objective optimisation, computer aided design, regional, business, high-transonic, low-supersonic.